[Bug c/66970] Add __has_builtin() macro

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66970

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Julian Taylor from comment #3)
> Similarly useful would be the related __has_attribute, those especially tend
> to cause a huge mess of version number checks.

That's already supported: https://gcc.gnu.org/gcc-5/changes.html