cmake: append to COMPILE_FLAGS property

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index bc4510a..5867e5b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2004,7 +2004,7 @@
       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>