[Bug libstdc++/79147] Useless _GLIBCXX_PACKAGE_* macros in <bits/c++config.h>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79147

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We might want to comment out STDC_HEADERS as well, instead of transforming it
to _GLIBCXX_STDC_HEADERS