macOS/iOS: Building app against lower SDK than Qt was built with should error at qmake stage

For static builds of Qt on macOS/iOS, we build with the appropriate guards in the code to only include calls to functions that are available in the SDK we're building against, but what we produce is then a static library, which is then linked into the final application by the user. The user may then use an older SDK which doesn't have the required functions. We should store somewhere which SDK version Qt was built with, and error out at qmake time if the user tries to build his app with an earlier SDK.