Verify supported Mac OS and Xcode version is used in build.rs

[repi]

Currently one get a cryptic Objective C compile error if one is building with an unsupported Xcode, and with PR #26 we now require Xcode 12 (I think?) so more common to run into this. Would be much nicer if we can detect unsupported Xcode version in build.rs and fail there before starting the slow MoltenVK build.

Spawned off from #28 (comment)

Something you could take a quick look at @MarnixKuijs ?

[MarnixKuijs]

Xcode 11 should still work. It at least did when I build with it (I have upgraded to 12 since then). It has mostly to do with the OS version your mac is running. With MoltenVK 1.1.0 its at least 10.15.4 afaik. IOS has a different version requirement

The problem I faced when I tried to add better error handling is that there is no easy way to get the version of IOS so the error handling would only work for MacOS.