[Bug c/66970] Add __has_builtin() macro

[egallager at gcc dot gnu.org]

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

--- Comment #14 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #13)
> Let me look into this request for GCC 10.

It's GCC 10 now.