Xcode expects the base SDK to always be the device SDK, so we can't
pass QMAKE_MAC_SDK on directly.
We use the Xcode SUPPORTED_PLATFORMS setting to inform Xcode about
which targets we can actually build and run for.
Change-Id: I32f474a9f2016fb410225cfef1fecc6598ac6c82
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>