add missing quotes to the fallback CHECK_OBJC_SOURCE_COMPILES macro

Closes:  https://github.com/libsdl-org/SDL/issues/6268 .

cmake/macros.cmake

@@ -96,7 +96,7 @@ if(${CMAKE_VERSION} VERSION_LESS "3.16.0")
   macro(CHECK_OBJC_SOURCE_COMPILES SOURCE VAR)
     set(PREV_REQUIRED_DEFS "${CMAKE_REQUIRED_DEFINITIONS}")
     set(CMAKE_REQUIRED_DEFINITIONS "-x objective-c ${PREV_REQUIRED_DEFS}")
-    CHECK_C_SOURCE_COMPILES(${SOURCE} ${VAR})
+    CHECK_C_SOURCE_COMPILES("${SOURCE}" ${VAR})
     set(CMAKE_REQUIRED_DEFINITIONS "${PREV_REQUIRED_DEFS}")
   endmacro()
 else()