CMake: Fixed build on Mac OS X.

CMakeLists.txt

@@ -991,6 +991,7 @@ elseif(APPLE)
   if(SDL_FILESYSTEM)
     set(SDL_FILESYSTEM_COCOA 1)
     file(GLOB FILESYSTEM_SOURCES ${SDL2_SOURCE_DIR}/src/filesystem/cocoa/*.m)
+    set_source_files_properties(${FILESYSTEM_SOURCES} PROPERTIES LANGUAGE C)
     set(SOURCE_FILES ${SOURCE_FILES} ${FILESYSTEM_SOURCES})
     set(HAVE_SDL_FILESYSTEM TRUE)
   endif()