build: Replace MAC_OSX macro with existing __APPLE__

Adopting `__APPLE__` aligns our project with broader industry practices, including those in prominent projects such as the Linux kernel (and even our own code).

See: https://sourceforge.net/p/predef/wiki/OperatingSystems/#macos

CMakeLists.txt

@@ -324,10 +324,7 @@ if (CMAKE_SYSTEM_NAME STREQUAL "Linux" AND CMAKE_SIZEOF_VOID_P EQUAL 4)
 endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
-  target_compile_definitions(core_interface INTERFACE
-    MAC_OSX
-    OBJC_OLD_DISPATCH_PROTOTYPES=0
-  )
+  target_compile_definitions(core_interface INTERFACE OBJC_OLD_DISPATCH_PROTOTYPES=0)
   # These flags are specific to ld64, and may cause issues with other linkers.
   # For example: GNU ld will interpret -dead_strip as -de and then try and use
   # "ad_strip" as the symbol for the entry point.