Unlike other platforms' definitions, the GGP SDK expects the project files to
define this instead of having the compiler define it for us.

Bug: angleproject:4237
Change-Id: I8f10311fdff785a74c5e49fc53a512f1c050bff3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2021010Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>