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

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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

--- Comment #17 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The problem is that gfortran development is dependent on a gcc-wide build mode
which affects more than just gfortran. There's no good reason that libstdc++
should be a blocker for you (for the record, the -Wabi flag has been present
for years, and the -Werror consequence of maintainer mode has been there for
years too, what changed was not in libstdc++ but g++).

If you had (for example) a --enable-gfortran-maintainer-mode option that
triggered the m4 regeneration then you wouldn't need to alter the build options
for the rest of gcc.

Or a makefile target that regenerated the files, without needing to enable
maintainer mode globally.

Or some other solution that was under your control and specific to your
subsystem, that didn't depend on other components that you don't need to depend
on.

That aside, I think not setting -Werror in libstdc++ for maintainer mode is
better than simply removing warning options. But I'm on holiday and not going
to commit anything myself, so do whatever you need to do. I'll put -Wabi=N back
later if we decide it should be there, with a suitable value of N so it doesn't
give the current warning.