Hide __has_attribute from non-clang compilers

gcc 4.9 doesn't know __has_attribute so don't expose it
for non-clang compilers

Change-Id: I991af712adbd56d64938a35b75b7de468154e3a6
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2553421
Tested-by: Patrick Georgi <pgeorgi@chromium.org>
Commit-Queue: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>