apples, pears etc: how to use __builtin_availability() elsewhere?

Hi,

clang has this nice runtime OS version checker but I cannot seem to find if this is useable anywhere except on Apple OSes.
For ObjC (and thus @available()) one can still argue that the language isn't of much use anywhere else, but that (lousy) argument doesn't fly for__builtin_available() .

Is there a list of known other platforms somewhere, or else is there at least a token that makes the test return false when the current platform is not listed?

If not, how is that NOT a bug?

Cheers,
R.

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