[Bug c/66970] Add __has_builtin() macro

[linux at carewolf dot com]

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

--- Comment #19 from Allan Jensen <linux at carewolf dot com> ---
(In reply to felix from comment #18)
> So even if this feature is adopted as-is, it will necessitate some changes
> in the documentation. And while I can sympathise with claims that this
> behaviour is surprising, what are the alternatives? If keywords should count
> as built-ins, should __has_builtin(sizeof) expand to 1? Should
> __has_builtin(volatile)?

No just keywords that begin with __builtin_..