Check GCC for __has_attribute 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>