[CMake] Detecting Mac OS X Version

[E. Wing]

I'm wondering the same about this topic. I think Sean might be right
about checking for the SDK target version instead of OS X version,
though I could come up with a contrived example where knowing the
actual OS X version could be important, such as a CMake bug/issue you
need to work around that only appears on 10.4.3 or something. So it
would be good to have both.

A warning about Darwin version checking...don't consider it a reliable
indicator about which OS X version you are using. It is possible for a
developer to upgrade Darwin independently from OS X. I remember seeing
somebody do this before because they needed some feature that wasn't
in the Apple shipping version. I think you should only check Darwin
versions if you are actually dealing directly with Darwin.

-Eric