Don't set USE_ICON on macOS; the icon is derived from the bundle

cmake/platforms/linux.cmake

@@ -4,6 +4,9 @@ if(NOT UNIX OR APPLE)
     return()
 endif()
 
+list(APPEND CLIENT_DEFINITIONS USE_ICON)
+list(APPEND RENDERER_DEFINITIONS USE_ICON)
+
 if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
     set_property(CACHE CMAKE_INSTALL_PREFIX PROPERTY VALUE /opt/quake3)
 endif()

cmake/platforms/unix.cmake

@@ -20,6 +20,3 @@ list(APPEND COMMON_LIBRARIES
     dl  # Dynamic loader
     m   # Math library
 )
-
-list(APPEND CLIENT_DEFINITIONS USE_ICON)
-list(APPEND RENDERER_DEFINITIONS USE_ICON)