The __has_attribute macro was one of improvements in GCC 5 release.
See https://gcc.gnu.org/gcc-5/changes.html.
This checks to make it work with older GCC.

BUG=angleproject:2119

Change-Id: I751249b9d028d1322829bebc1ca8d423edf27057
Reviewed-on: https://chromium-review.googlesource.com/599152
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>