cmake: fix Metal detection

Confirmed to work on:
macOS 10.15.7
XCode Version 12.4 (12D4e)

CMakeLists.txt

@@ -1992,21 +1992,15 @@ elseif(APPLE)
     endif()
 
     if(SDL_VULKAN OR SDL_METAL OR SDL_RENDER_METAL)
-      set(ORIG_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-      set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -ObjC")
-      check_c_source_compiles("
+      check_objc_source_compiles("
         #include <AvailabilityMacros.h>
         #import <Metal/Metal.h>
         #import <QuartzCore/CAMetalLayer.h>
 
         #if (!TARGET_CPU_X86_64 && !TARGET_CPU_ARM64)
         #error Metal doesn't work on this configuration
         #endif
-        int main(void) {
-            return 0;
-        }
-        " HAVE_FRAMEWORK_METAL)
-      set(CMAKE_REQUIRED_FLAGS ${ORIG_CMAKE_REQUIRED_FLAGS})
+        int main(void) {}" HAVE_FRAMEWORK_METAL)
       if(HAVE_FRAMEWORK_METAL)
         set(SDL_FRAMEWORK_METAL 1)
         set(SDL_FRAMEWORK_QUARTZCORE 1)