[Bug libstdc++/86450] Bootstrap failure due to -Wabi

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86450

--- Comment #23 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This should be fixed now, please confirm (I can't even get a build to complete
with --enable-maintainer-mode, I continue to be amazed you rely on something so
fragile).

--enable-maintainer-mode no longer implies -Werror for libstdc++. To get
-Werror you need to use --enable-werror (as one would expect).

The -Wabi warnings are fixed by using -Wabi=2 to warn about ABI changes since
-fabi-version=2 (and not by removing the option entirely).