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

[fw at gcc dot gnu.org]

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

--- Comment #7 from Florian Weimer <fw at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #5)
> Would an ugly but pragmatic approach be possible? e.g. if the first line of
> the translation unit is "/* confdefs.h */ then assume GCC is being invoked
> by configure, and only warn instead of giving an error.

As far as ugly hacks go, examining the file name is probably easier (I've only
seen conftest.c so far).