[Bug c/78829] New: bit-rotten "C99 mode" references in GCC manual

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78829

            Bug ID: 78829
           Summary: bit-rotten "C99 mode" references in GCC manual
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sandra at gcc dot gnu.org
  Target Milestone: ---

There are quite a few references in the GCC manual to "C99" or "C99 mode" when
it really is describing behavior common to C99 and later standards.  This is
really confusing now that GCC defaults to a dialect based on C11 -- the
documentation with the "C99" references was probably written when C90 was still
the default, and now "C99" refers to an old standard instead of a new one.  All
these instances need to be reviewed and corrected as necessary.

I also think that we could eliminate some material from extend.texi by just
having a single section with an abbreviated bullet list of C99 features
supported as extensions in GNU C90 mode (VLAs, variadic macros, // comments,
etc).  And then maybe group some other things into a section on backward
compatibility features like the alternate syntax for variadic macros (or maybe
we should even deprecate/remove the support for that).