MINOR: compiler: add a __has_builtin() macro to detect features more easily

MINOR: compiler: add a __has_builtin() macro to detect features more easily

We already have a __has_attribute() macro to detect when the compiler
supports a specific attribute, but we didn't have the equivalent for
builtins. clang-3 and gcc-10 have __has_builtin() for this. Let's just
bring it using the same mechanism as __has_attribute(), which will allow
us to simply define the macro's value for older compilers. It will save
us from keeping that many compiler-specific tests that are incomplete
(e.g. the __builtin_unreachable() test currently doesn't cover clang).

(cherry picked from commit 96cfcb1df36fdf8fb963371ad5e167c733d842b1)
Signed-off-by: Willy Tarreau <w@1wt.eu>