Catalina SDK clang stopped automatically searching the SDK include paths when
invoked without --sysroot:

bitcoin#16367 (comment)
Homebrew/homebrew-core#45061

This hasn't been a problem for current native depends packages because are
passing their own --sysroot values, and hasn't been a problem for current host
packages because they use `darwin_` commands instead of `build_darwin_`
commands.  But the current `build_darwin_CC` and `build_darwin_CXX` commands
are still unnecessarily fragile, and incompatible with new native depends
packages added in bitcoin#18677.

Cory Fields <[email protected]> suggested in
bitcoin#16367 (comment) switching
compiler from SDK clang to native clang (from $PATH) to avoid this problem.
This is easy and makes a certain amount of sense for building native packages,
as opposed to host packages. But fanquake <[email protected]> pointed out in
bitcoin#18677 (comment) that it
would be inconsistent use switch to non-SDK compilers while still using other
SDK tools like ranlib and install_name_tool. So simplest, minimal fix seems to
be just adding the missing --sysroot option.

Starting with the Clang shipped with Xcode 12, Apple has enabled
-Werror=implicit-function-declaration by default. This causes bdbs mutex
detection to fail when building on macOS (not cross-compiling):

checking for mutexes... UNIX/fcntl
configure: WARNING: NO SHARED LATCH IMPLEMENTATION FOUND FOR THIS PLATFORM.
configure: error: Unable to find a mutex implementation

as previously emitted warnings are being turned into errors. i.e:

error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]

Append -Wno-error=implicit-function-declaration to cflags so that
-Wimplicit-function-declaration returns to being a warning, and the
configure checks will succeed.

Fixes bitcoin#19411.