[Bug c/91092] New: Error on implicit function declarations by default

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91092

            Bug ID: 91092
           Summary: Error on implicit function declarations by default
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fw at gcc dot gnu.org
  Target Milestone: ---

Once configure scripts have been cleaned up (and we have verified this for
Fedora and perhaps Debian), c99/gnu99/c11/gnu11 mode should all default to
emitting errors for implicit function declarations.

Implicit function declarations were removed from C99, but have been kept in GCC
so far, mainly for compatibility with existing configure checks (which give
wrong results if implicit function declarations are used in tests).