cmake: append to COMPILE_FLAGS property

CMakeLists.txt

@@ -2004,7 +2004,7 @@ elseif(APPLE)
       set(SDL_FRAMEWORK_COREHAPTICS 1)
     else()
       file(GLOB JOYSTICK_SOURCES ${SDL2_SOURCE_DIR}/src/joystick/darwin/*.c)
-      set_source_files_properties(${MFI_JOYSTICK_SOURCES} PROPERTIES COMPILE_FLAGS -fobjc-weak)
+      set_property(SOURCE ${MFI_JOYSTICK_SOURCES} APPEND_STRING PROPERTY COMPILE_FLAGS " -fobjc-weak")
       check_objc_source_compiles("
           #include <AvailabilityMacros.h>
           #include <TargetConditionals.h>