-MAC_OS Compiling for some version of Mac OS?
-MAC_OS8 Compiling for Mac OS version 8. Requires MAC_OS?
-MAC_OSX Compiling for Mac OS X? Is that also valid for Darwin?
-MAC_OS_X ?? Apparently only used once in mac.c.
+MAC_OS Compiling for the `mac' window-system.
+MAC_OS8 Compiling for Mac OS Classic (v8 or v9). No longer supported, all code using it can be removed.
+MAC_OSX Compiling for Mac OS X. Not bare Darwin.