The __clang__ macro cannot be directly used with an #if directive if it
has not been defined. This causes a compilation error under GCC 5.4
Linux.

Change-Id: Ieca752cd9f0e90c82febc35e19e1c1f3c6096de1
Reviewed-on: https://chromium-review.googlesource.com/397220Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>