Add has_macosx_plist_overrides? method

This commit adds the has_macosx_plist_overrides? method to determine
whether we're on a version of OS X >= 10.6. Plist handling changed
as of version 10.6, and this method allows us to implement different
plist behavior without needing to modify code whenever Apple releases
a new version of OS X (assuming plist behavior doesn't change).