Support gnu standard versions for querying compiler

Hi, I am receiving the error `identifier "CLOCK_MONOTONIC" is undefined` from cpptools for the below code, which I don't receive when compiling with gcc. If I include `"defines": ["_POSIX_C_SOURCE 199309L"]` in `c_cpp_properties.json` then the issue is resolved and cpptools doesn't complain.

My question is, why is this behavior different when I compile with gcc? I believe the default C standard for gcc is gnu11, which doesn't require `_POSIX_C_SOURCE` to be defined. Is cpptools using a different standard, and if so is there a way to force using gnu11? I've tried adding `-std=gnu11` to the compiler path, but that doesn't seem to solve the issue.

- OS and Version: Ubuntu 16.04.3 LTS
- VS Code Version: 1.28.2
- C/C++ Extension Version: 0.20.1
- Other extensions: none

```c
#include <stdint.h>
#include <time.h>

uint64_t timer_diff(struct timespec *start)
{
	uint64_t diff;
	struct timespec now;

	clock_gettime(CLOCK_MONOTONIC, &now);

	diff = UINT64_C(1000000000) * (now.tv_sec - start->tv_sec) +
		   now.tv_nsec - start->tv_nsec;

	return diff;
}
```

```json
{
    "configurations": [
        {
            "name": "Linux",
            "includePath": [
                "${workspaceFolder}/**"
            ],
            "compilerPath": "/usr/lib/gcc",
            "intelliSenseMode": "gcc-x64",
            "browse": {
                "path": [
                    "${workspaceFolder}/**"
                ],
                "limitSymbolsToIncludedHeaders": true,
                "databaseFilename": ""
            },
            "cStandard": "c11",
            "cppStandard": "c++17"
        }
    ],
    "version": 4
}
```

```
initialized
workspace/didChangeConfiguration
IntelliSense Engine = Default.
The extension will use the Tag Parser for IntelliSense when #includes don't resolve.
Autocomplete is enabled.
Error squiggles are enabled.
File exclude: **/.git
File exclude: **/.svn
File exclude: **/.hg
File exclude: **/CVS
File exclude: **/.DS_Store
File exclude: **/.vscode
Search exclude: **/node_modules
Search exclude: **/bower_components
Search exclude: **/.vscode
textDocument/didOpen
cpptools/queryCompilerDefaults
Attempting to get defaults from compiler found on the machine: '/usr/bin/gcc'
terminating child process: 5243
cpptools/activeDocumentChange
cpptools/textEditorSelectionChange
cpptools/didChangeFolderSettings
Attempting to get defaults from compiler found on the machine: '/usr/bin/gcc'
Code browsing service initialized
Attempting to get defaults from compiler found on the machine: '/usr/bin/gcc'
  Folder: /usr/lib/gcc/x86_64-linux-gnu/5/include/ will be indexed
  Folder: /usr/local/include/ will be indexed
  Folder: /usr/lib/gcc/x86_64-linux-gnu/5/include-fixed/ will be indexed
  Folder: /usr/include/ will be indexed
  Folder: /home/dan/Desktop/vscode-test/ will be indexed
textDocument/didOpen
Discovering files...
  Processing folder (recursive): /usr/lib/gcc/x86_64-linux-gnu/5/include/
  Processing folder (recursive): /usr/local/include/
  Processing folder (recursive): /usr/lib/gcc/x86_64-linux-gnu/5/include-fixed/
  Processing folder (recursive): /usr/include/
textDocument/documentSymbol
textDocument/codeAction
sending compilation args for /home/dan/Desktop/vscode-test/main.c
  include: /usr/lib/gcc/x86_64-linux-gnu/5/include
  include: /usr/local/include
  include: /usr/lib/gcc/x86_64-linux-gnu/5/include-fixed
  include: /usr/include/x86_64-linux-gnu
  include: /usr/include
  define: __STDC__=1
  define: __STDC_VERSION__=201112L
  define: __STDC_UTF_16__=1
  define: __STDC_UTF_32__=1
  define: __STDC_HOSTED__=1
  define: __GNUC__=5
  define: __GNUC_MINOR__=4
  define: __GNUC_PATCHLEVEL__=0
  define: __VERSION__="5.4.0 20160609"
  define: __ATOMIC_RELAXED=0
  define: __ATOMIC_SEQ_CST=5
  define: __ATOMIC_ACQUIRE=2
  define: __ATOMIC_RELEASE=3
  define: __ATOMIC_ACQ_REL=4
  define: __ATOMIC_CONSUME=1
  define: __FINITE_MATH_ONLY__=0
  define: _LP64=1
  define: __LP64__=1
  define: __SIZEOF_INT__=4
  define: __SIZEOF_LONG__=8
  define: __SIZEOF_LONG_LONG__=8
  define: __SIZEOF_SHORT__=2
  define: __SIZEOF_FLOAT__=4
  define: __SIZEOF_DOUBLE__=8
  define: __SIZEOF_LONG_DOUBLE__=16
  define: __SIZEOF_SIZE_T__=8
  define: __CHAR_BIT__=8
  define: __BIGGEST_ALIGNMENT__=16
  define: __ORDER_LITTLE_ENDIAN__=1234
  define: __ORDER_BIG_ENDIAN__=4321
  define: __ORDER_PDP_ENDIAN__=3412
  define: __BYTE_ORDER__=__ORDER_LITTLE_ENDIAN__
  define: __FLOAT_WORD_ORDER__=__ORDER_LITTLE_ENDIAN__
  define: __SIZEOF_POINTER__=8
  define: __SIZE_TYPE__=long unsigned int
  define: __PTRDIFF_TYPE__=long int
  define: __WCHAR_TYPE__=int
  define: __WINT_TYPE__=unsigned int
  define: __INTMAX_TYPE__=long int
  define: __UINTMAX_TYPE__=long unsigned int
  define: __CHAR16_TYPE__=short unsigned int
  define: __CHAR32_TYPE__=unsigned int
  define: __SIG_ATOMIC_TYPE__=int
  define: __INT8_TYPE__=signed char
  define: __INT16_TYPE__=short int
  define: __INT32_TYPE__=int
  define: __INT64_TYPE__=long int
  define: __UINT8_TYPE__=unsigned char
  define: __UINT16_TYPE__=short unsigned int
  define: __UINT32_TYPE__=unsigned int
  define: __UINT64_TYPE__=long unsigned int
  define: __INT_LEAST8_TYPE__=signed char
  define: __INT_LEAST16_TYPE__=short int
  define: __INT_LEAST32_TYPE__=int
  define: __INT_LEAST64_TYPE__=long int
  define: __UINT_LEAST8_TYPE__=unsigned char
  define: __UINT_LEAST16_TYPE__=short unsigned int
  define: __UINT_LEAST32_TYPE__=unsigned int
  define: __UINT_LEAST64_TYPE__=long unsigned int
  define: __INT_FAST8_TYPE__=signed char
  define: __INT_FAST16_TYPE__=long int
  define: __INT_FAST32_TYPE__=long int
  define: __INT_FAST64_TYPE__=long int
  define: __UINT_FAST8_TYPE__=unsigned char
  define: __UINT_FAST16_TYPE__=long unsigned int
  define: __UINT_FAST32_TYPE__=long unsigned int
  define: __UINT_FAST64_TYPE__=long unsigned int
  define: __INTPTR_TYPE__=long int
  define: __UINTPTR_TYPE__=long unsigned int
  define: __has_include(STR)=__has_include__(STR)
  define: __has_include_next(STR)=__has_include_next__(STR)
  define: __GXX_ABI_VERSION=1009
  define: __SCHAR_MAX__=0x7f
  define: __SHRT_MAX__=0x7fff
  define: __INT_MAX__=0x7fffffff
  define: __LONG_MAX__=0x7fffffffffffffffL
  define: __LONG_LONG_MAX__=0x7fffffffffffffffLL
  define: __WCHAR_MAX__=0x7fffffff
  define: __WCHAR_MIN__=(-__WCHAR_MAX__ - 1)
  define: __WINT_MAX__=0xffffffffU
  define: __WINT_MIN__=0U
  define: __PTRDIFF_MAX__=0x7fffffffffffffffL
  define: __SIZE_MAX__=0xffffffffffffffffUL
  define: __INTMAX_MAX__=0x7fffffffffffffffL
  define: __INTMAX_C(c)=c ## L
  define: __UINTMAX_MAX__=0xffffffffffffffffUL
  define: __UINTMAX_C(c)=c ## UL
  define: __SIG_ATOMIC_MAX__=0x7fffffff
  define: __SIG_ATOMIC_MIN__=(-__SIG_ATOMIC_MAX__ - 1)
  define: __INT8_MAX__=0x7f
  define: __INT16_MAX__=0x7fff
  define: __INT32_MAX__=0x7fffffff
  define: __INT64_MAX__=0x7fffffffffffffffL
  define: __UINT8_MAX__=0xff
  define: __UINT16_MAX__=0xffff
  define: __UINT32_MAX__=0xffffffffU
  define: __UINT64_MAX__=0xffffffffffffffffUL
  define: __INT_LEAST8_MAX__=0x7f
  define: __INT8_C(c)=c
  define: __INT_LEAST16_MAX__=0x7fff
  define: __INT16_C(c)=c
  define: __INT_LEAST32_MAX__=0x7fffffff
  define: __INT32_C(c)=c
  define: __INT_LEAST64_MAX__=0x7fffffffffffffffL
  define: __INT64_C(c)=c ## L
  define: __UINT_LEAST8_MAX__=0xff
  define: __UINT8_C(c)=c
  define: __UINT_LEAST16_MAX__=0xffff
  define: __UINT16_C(c)=c
  define: __UINT_LEAST32_MAX__=0xffffffffU
  define: __UINT32_C(c)=c ## U
  define: __UINT_LEAST64_MAX__=0xffffffffffffffffUL
  define: __UINT64_C(c)=c ## UL
  define: __INT_FAST8_MAX__=0x7f
  define: __INT_FAST16_MAX__=0x7fffffffffffffffL
  define: __INT_FAST32_MAX__=0x7fffffffffffffffL
  define: __INT_FAST64_MAX__=0x7fffffffffffffffL
  define: __UINT_FAST8_MAX__=0xff
  define: __UINT_FAST16_MAX__=0xffffffffffffffffUL
  define: __UINT_FAST32_MAX__=0xffffffffffffffffUL
  define: __UINT_FAST64_MAX__=0xffffffffffffffffUL
  define: __INTPTR_MAX__=0x7fffffffffffffffL
  define: __UINTPTR_MAX__=0xffffffffffffffffUL
  define: __GCC_IEC_559=2
  define: __GCC_IEC_559_COMPLEX=2
  define: __FLT_EVAL_METHOD__=0
  define: __DEC_EVAL_METHOD__=2
  define: __FLT_RADIX__=2
  define: __FLT_MANT_DIG__=24
  define: __FLT_DIG__=6
  define: __FLT_MIN_EXP__=(-125)
  define: __FLT_MIN_10_EXP__=(-37)
  define: __FLT_MAX_EXP__=128
  define: __FLT_MAX_10_EXP__=38
  define: __FLT_DECIMAL_DIG__=9
  define: __FLT_MAX__=3.40282346638528859812e+38F
  define: __FLT_MIN__=1.17549435082228750797e-38F
  define: __FLT_EPSILON__=1.19209289550781250000e-7F
  define: __FLT_DENORM_MIN__=1.40129846432481707092e-45F
  define: __FLT_HAS_DENORM__=1
  define: __FLT_HAS_INFINITY__=1
  define: __FLT_HAS_QUIET_NAN__=1
  define: __DBL_MANT_DIG__=53
  define: __DBL_DIG__=15
  define: __DBL_MIN_EXP__=(-1021)
  define: __DBL_MIN_10_EXP__=(-307)
  define: __DBL_MAX_EXP__=1024
  define: __DBL_MAX_10_EXP__=308
  define: __DBL_DECIMAL_DIG__=17
  define: __DBL_MAX__=((double)1.79769313486231570815e+308L)
  define: __DBL_MIN__=((double)2.22507385850720138309e-308L)
  define: __DBL_EPSILON__=((double)2.22044604925031308085e-16L)
  define: __DBL_DENORM_MIN__=((double)4.94065645841246544177e-324L)
  define: __DBL_HAS_DENORM__=1
  define: __DBL_HAS_INFINITY__=1
  define: __DBL_HAS_QUIET_NAN__=1
  define: __LDBL_MANT_DIG__=64
  define: __LDBL_DIG__=18
  define: __LDBL_MIN_EXP__=(-16381)
  define: __LDBL_MIN_10_EXP__=(-4931)
  define: __LDBL_MAX_EXP__=16384
  define: __LDBL_MAX_10_EXP__=4932
  define: __DECIMAL_DIG__=21
  define: __LDBL_MAX__=1.18973149535723176502e+4932L
  define: __LDBL_MIN__=3.36210314311209350626e-4932L
  define: __LDBL_EPSILON__=1.08420217248550443401e-19L
  define: __LDBL_DENORM_MIN__=3.64519953188247460253e-4951L
  define: __LDBL_HAS_DENORM__=1
  define: __LDBL_HAS_INFINITY__=1
  define: __LDBL_HAS_QUIET_NAN__=1
  define: __DEC32_MANT_DIG__=7
  define: __DEC32_MIN_EXP__=(-94)
  define: __DEC32_MAX_EXP__=97
  define: __DEC32_MIN__=1E-95DF
  define: __DEC32_MAX__=9.999999E96DF
  define: __DEC32_EPSILON__=1E-6DF
  define: __DEC32_SUBNORMAL_MIN__=0.000001E-95DF
  define: __DEC64_MANT_DIG__=16
  define: __DEC64_MIN_EXP__=(-382)
  define: __DEC64_MAX_EXP__=385
  define: __DEC64_MIN__=1E-383DD
  define: __DEC64_MAX__=9.999999999999999E384DD
  define: __DEC64_EPSILON__=1E-15DD
  define: __DEC64_SUBNORMAL_MIN__=0.000000000000001E-383DD
  define: __DEC128_MANT_DIG__=34
  define: __DEC128_MIN_EXP__=(-6142)
  define: __DEC128_MAX_EXP__=6145
  define: __DEC128_MIN__=1E-6143DL
  define: __DEC128_MAX__=9.999999999999999999999999999999999E6144DL
  define: __DEC128_EPSILON__=1E-33DL
  define: __DEC128_SUBNORMAL_MIN__=0.000000000000000000000000000000001E-6143DL
  define: __REGISTER_PREFIX__=
  define: __USER_LABEL_PREFIX__=
  define: __GNUC_STDC_INLINE__=1
  define: __NO_INLINE__=1
  define: __STRICT_ANSI__=1
  define: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1=1
  define: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2=1
  define: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4=1
  define: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8=1
  define: __GCC_ATOMIC_BOOL_LOCK_FREE=2
  define: __GCC_ATOMIC_CHAR_LOCK_FREE=2
  define: __GCC_ATOMIC_CHAR16_T_LOCK_FREE=2
  define: __GCC_ATOMIC_CHAR32_T_LOCK_FREE=2
  define: __GCC_ATOMIC_WCHAR_T_LOCK_FREE=2
  define: __GCC_ATOMIC_SHORT_LOCK_FREE=2
  define: __GCC_ATOMIC_INT_LOCK_FREE=2
  define: __GCC_ATOMIC_LONG_LOCK_FREE=2
  define: __GCC_ATOMIC_LLONG_LOCK_FREE=2
  define: __GCC_ATOMIC_TEST_AND_SET_TRUEVAL=1
  define: __GCC_ATOMIC_POINTER_LOCK_FREE=2
  define: __GCC_HAVE_DWARF2_CFI_ASM=1
  define: __PRAGMA_REDEFINE_EXTNAME=1
  define: __SSP_STRONG__=3
  define: __SIZEOF_INT128__=16
  define: __SIZEOF_WCHAR_T__=4
  define: __SIZEOF_WINT_T__=4
  define: __SIZEOF_PTRDIFF_T__=8
  define: __amd64=1
  define: __amd64__=1
  define: __x86_64=1
  define: __x86_64__=1
  define: __SIZEOF_FLOAT80__=16
  define: __SIZEOF_FLOAT128__=16
  define: __ATOMIC_HLE_ACQUIRE=65536
  define: __ATOMIC_HLE_RELEASE=131072
  define: __k8=1
  define: __k8__=1
  define: __code_model_small__=1
  define: __MMX__=1
  define: __SSE__=1
  define: __SSE2__=1
  define: __FXSR__=1
  define: __SSE_MATH__=1
  define: __SSE2_MATH__=1
  define: __gnu_linux__=1
  define: __linux=1
  define: __linux__=1
  define: __unix=1
  define: __unix__=1
  define: __ELF__=1
  define: __DECIMAL_BID_FORMAT__=1
  define: _STDC_PREDEF_H=1
  define: __STDC_IEC_559__=1
  define: __STDC_IEC_559_COMPLEX__=1
  define: __STDC_ISO_10646__=201505L
  define: __STDC_NO_THREADS__=1
  other: --gcc
  other: --gnu_version=50400
  stdver: --c11
  intelliSenseMode: gcc
Checking for syntax errors: file:///home/dan/Desktop/vscode-test/main.c
queue_update_intellisense for files in tu of: /home/dan/Desktop/vscode-test/main.c
  Processing folder (recursive): /home/dan/Desktop/vscode-test/
  Discovering files: 3870 file(s) processed
  0 file(s) removed from database
Done discovering files.
Populate include completion cache.
Parsing remaining files...
  Parsing: 0 files(s) processed
Done parsing remaining files.
terminating child process: 5258
errorSquiggles count: 1
textDocument/codeAction
textDocument/hover
textDocument/hover
cpptools/textEditorSelectionChange
textDocument/codeAction
cpptools/activeDocumentChange
cpptools/textEditorSelectionChange
cpptools/textEditorSelectionChange
textDocument/codeAction
```

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Support gnu standard versions for querying compiler #2782

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Support gnu standard versions for querying compiler #2782

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