The proper way to do this is to test for __APPLE__ and __MACH__, where
__APPLE__ tests for an Apple OS and __MACH__ tests that it is based on CMU
Mach. Using both tests ensures that just Darwin (Apple's open source base
for MacOS, iOS, etc.) is recognized. __APPLE__ by itself will test for any
Apple OS, which can include older OS 7-9 and any future Apple OS. __MACH__
tests for any OS based on CMU Mach, including Darwin and GNU Hurd.
Fixes#7232.
