[Bug c/83656] missing -Wbuiltin-declaration-mismatch on declaration without prototype

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83656

--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
Revised patch that should resolve the autoconf concerns:
  https://gcc.gnu.org/ml/gcc-patches/2018-11/msg00120.html