There has been a problem with the OS_MACOSX #define for a while. OS_MACOSX means "Mac or iOS", which is inherently confusing. Given that Apple moved away from the "Mac OS X" appellation a while ago and is now moving away from the 10.x version number, the Mac team is happy to announce that we are migrating away from the OS_MACOSX macro.
It is replaced by two macros: OS_MAC, which is defined only on the Mac (not iOS), and OS_APPLE, which is a direct replacement for OS_MACOSX as it is defined for both Mac and iOS.
I will be migrating the existing macros over the next few days.