This turns previous compiler errors complaining about missing files
into proper ./configure time errors telling the user which version
of boost is required.

And probably other Linux distributions with long-term support releases.

Also update manual stating what version is needed;
I checked that 1.14 is the oldest version with which current nix compiles,
and added autoconf feature checks for some functions added in that release
that nix uses.

And probably many other distributions.

Until now, ./configure would fail silently printing a warning

    ./configure: line 4621: AX_CXX_COMPILE_STDCXX_17: command not found

and then continuing, later failing with a C++ #error saying that some C++11
feature isn't supported (it didn't even get to the C++17 features).

This is because older distributions don't come with the
`AX_CXX_COMPILE_STDCXX_17` m4 macro.

This commit vendors that macro accordingly.

Now ./configure complains correctly:

    configure: error: *** A compiler with support for C++17 language features is required.

On Ubuntu 16.04, ./configure completes if a newer compiler is used, e.g. with
gcc-7 from https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test
using:

    ./bootstrap.sh
    ./configure CXX=g++-7 --disable-doc-gen --with-boost=$(nix-build --no-link '<nixpkgs>' -A boost.dev)

autotools-based systems usually allow user to
append own LDFLAGS like
    LDFLAGS="-Wl,-O1 -Wl,--as-needed -Wl,--hash-style=gnu"
at ./configure stage

This change plumbs LDFLAGS through similar to existing CXXFLAGS variable.

Signed-off-by: Sergei Trofimovich <[email protected]>

That was incorrect, because checking the dirent type already requires
a working compiler.

It had the effect that setting e.g. `: ${CFLAGS=""}` before `AC_PROG_CC`
as per `AC_PROG_CC`'s documentation would have no effect, because
`AC_STRUCT_DIRENT_D_TYPE` would automatically set CFLASGS.

(In a followup commit `: ${CFLAGS=""}` will be used, so it's important
to get this working first.)

As is normal for autoconf-based projects.

For example, it is a common use case to do

    ./configure CXXFLAGS=-O0

This did not work for nix until now, because the `CXXFLAGS=` declaration
would unconditionally erase what the user had specified.

The custom `OPTIMIZE` flag is removed, but the default `-O3` is retained;
autoconf would default to `-g -O2` by default otherwise as documented on:

https://www.gnu.org/software/autoconf/manual/autoconf-2.60/html_node/C-Compiler.html
https://www.gnu.org/software/autoconf/manual/autoconf-2.60/html_node/C_002b_002b-Compiler.html

It was forgotten to be removed with
commit c5f23f1
and so it until now stayed unsubstituted as `HAVE_READLINE = @HAVE_READLINE@`
in Makefile.config.

For example, Ubuntu 16.04 and many similar long-term-support distros
have older versions.

Our use of boost::coroutine2 depends on -lboost_context,
which in turn depends on `-lboost_thread`, which in turn depends
on `-lboost_system`.

I suspect that this builds on nix only because of low-level hacks
like NIX_LDFLAGS.

This commit passes the proper linker flags, thus fixing bootstrap
builds on non-nix distributions like Ubuntu 16.04.

With these changes, I can build Nix on Ubuntu 16.04 using:

    ./bootstrap.sh
    ./configure --prefix=$HOME/editline-prefix \
      --disable-doc-gen \
      CXX=g++-7 \
      --with-boost=$HOME/boost-prefix \
      EDITLINE_CFLAGS=-I$HOME/editline-prefix/include \
      EDITLINE_LIBS=-leditline \
      LDFLAGS=-L$HOME/editline-prefix/lib
    make

where

* g++-7 comes from gcc-7 from
  https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test,
* editline 1.14 from https://github.com/troglobit/editline/releases/tag/1.14.0
	was installed into `$HOME/editline-prefix`
  (because Ubuntu 16.04's `editline` is too old to have the function nix uses),
* boost 1.66 from
	https://www.boost.org/doc/libs/1_66_0/more/getting_started/unix-variants.html
	was installed into $HOME/boost-prefix (because Ubuntu 16.04 only has 1.58)

Also give a helpful error message on what package the user likely
has to install to make it work.

This is to avoid confusion as in commit
a0d2904.

This should finally allow us to address all cases of build errors due to
differences between release tarballs and building from git.

See also NixOS#506 (comment)