libstdc++: Remove unwanted STDC_HEADERS macro from c++config.h [PR79147]

libstdc++: Remove unwanted STDC_HEADERS macro from c++config.h [PR79147]

Similar to r16-4034-g1953939243e1ab, this comments out another macro
that Autoconf adds to the generated config.h but which is not wanted in
the c++config.h file that we install.

There's no benefit to defining _GLIBCXX_STDC_HEADERS in user code, so we
should just prevent it from being defined.

libstdc++-v3/ChangeLog:

	PR libstdc++/79147
	PR libstdc++/103650
	* include/Makefile.am (c++config.h): Adjust sed command to
	comment out STDC_HEADERS macro.
	* include/Makefile.in: Regenerate.