APIs that are only available on newer macOS versions need to be
__builtin_available for a runtime check that won't load
the symbol at all on older OS versions.
Split out of !1108 (merged) because there are a lot of failures:
../gio/glocalfileinfo.c:2779:9: error: 'utimensat' is only available on macOS 10.13 or newer [-Werror,-Wunguarded-availability-new] res = utimensat (AT_FDCWD, filename, times_n, 0); ^~~~~~~~~