Update ax_cv_cxx_compile_cxx.m4

Update ax_cv_cxx_compile_cxx.m4

This file provides the AX_CXX_COMPILE_STDCXX macro.  In the context of
the following patch, I wanted to build and test GDB in c++17 mode.  The
version of the macro we have in the repo does not support detecting
c++17 compilers, but the upstream version has been updated to do so.

Since we have local modifications to the file, I had to reconcile our
modifications and the updated upstream version (which was relatively
straightforward).

gdb/ChangeLog:

	* ax_cxx_compile_stdcxx.m4: Sync with upstream.
	* configure: Re-generate.