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

[redi at gcc dot gnu.org]

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

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And it doesn't help all the existing configure scripts, that won't get
regenerated even if a new autoconf is released.