Do not assume __has_builtin is available 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>