The latest version of this change disables multithreaded D3D shader
compilation with any C++ compiler other than VC (e.g. Clang).

Change-Id: If3ef5fd650055ae66397ea2c9121bfcf1792892a
Reviewed-on: https://chromium-review.googlesource.com/250328Tested-by: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Cooper Partin <coopp@microsoft.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>