Fix GCC 5.4 compilation error when parsing the __clang__ macro
Fix GCC 5.4 compilation error when parsing the __clang__ macro

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/397220
Reviewed-by: Olli Etuaho <38ae6c0e2772a20e8e989f80cb8c99d274e44242@nvidia.com>
Reviewed-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Commit-Queue: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>

MSOpenTech/angle,ecoal95/angle,ppy/angle,MSOpenTech/angle,ecoal95/angle,MSOpenTech/angle,ppy/angle,ecoal95/angle,MSOpenTech/angle,ppy/angle,ppy/angle,ecoal95/angle,ecoal95/angle