The advantage of using this compiler builtin instead of our own platform
checks is that the compiler will warn us (if -Wunguarded-availability is
turned on for APIs introduced before 10.13 or by default for later ones)
if a check is forgotten, which is not the case for the manual checks.
Update the code to use WX_IS_MACOS_AVAILABLE() macro, which expands to
__builtin_available() when supported, and also use API_AVAILABLE() where
it makes sense to avoid having too many checks.
