Compilation with GCC breaks as there is a build check using it.
To avoid the problem, we declare the macro HAS_BUILTIN that maps
to __has_builtin if it is defined, or 0 otherwise.

Bug: chromium:819294
Change-Id: I105d566f08baa495fccd553d7b06eb8ce36621ef
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1713604
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>