Last check-in was a mistake...

I've been told apple gcc version number is not guaranteed to increase
monotonically. Change the preprocess condition to make it less risky.

The configuration change is done during the middle 10.4 life cycle so we have
to check __APPLE_CC. For future OS X release, we should be able to assume
-fenable-cxa-atexit is the default.

llvm-svn: 30024