Define angle_use_gbm=false when !angle_has_build angle_use_gbm is not defined when !angle_has_build. This causes build to fail. Define it to false in this case. Bug: angleproject:425569857 Change-Id: I8ba66f528b3052725467aeafe046900ec06b8989 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6701310 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Kramer Ge <fangzhoug@chromium.org>