Define MVK_MACCAT build macro and use it to conditionally compile code to align
with build features and capabilities of Mac Catalyst platform on macOS 11.0+.
Treat Mac Catalyst as minor variation of macOS 11.0.
Update documentation.
Currently only support Mac Catalyst on macOS 11.0+, to avoid complexities of
deselecting iOS features and capabilities for Mac Catalyst on previous macOS versions.
Mac Catalyst (and Simulators) require use of XCFrameworks.
Currently unable to generate a dylib for Mac Catalyst.
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