The ninja generation step could get confused if we try to build the
glslang wrapper project on platforms that don't support glslang. Turn
it off if the Metal / Vulkan back-ends are not enabled.

Bug: angleproject:4053
Change-Id: If4dd2856681f18f4f45e952bf88e33371b9a1e3c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1884010Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>