Prevent compilation with unsupported Apple SDK versions Bug: angleproject:360147119 Change-Id: Ie9924cb49ffa729d454a2869a2d19d957500bd10 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5793513 Reviewed-by: Kimmo Kinnunen <kkinnunen@apple.com> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>