cmake: added a FIXME for later.

Have to figure out what cmake version fixed this and bump the minimum to that.

CMakeLists.txt

@@ -1326,6 +1326,7 @@ elseif(APPLE)
   if(SDL_FILE)
     file(GLOB EXTRA_SOURCES ${SDL2_SOURCE_DIR}/src/file/cocoa/*.m)
     set(SOURCE_FILES ${EXTRA_SOURCES} ${SOURCE_FILES})
+    # !!! FIXME: modern CMake doesn't need "LANGUAGE C" for Objective-C.
     set_source_files_properties(${EXTRA_SOURCES} PROPERTIES LANGUAGE C)
     set(HAVE_SDL_FILE TRUE)
     set(SDL_FRAMEWORK_COCOA 1)
@@ -1396,6 +1397,7 @@ elseif(APPLE)
   if(SDL_FILESYSTEM)
     set(SDL_FILESYSTEM_COCOA 1)
     file(GLOB FILESYSTEM_SOURCES ${SDL2_SOURCE_DIR}/src/filesystem/cocoa/*.m)
+    # !!! FIXME: modern CMake doesn't need "LANGUAGE C" for Objective-C.
     set_source_files_properties(${FILESYSTEM_SOURCES} PROPERTIES LANGUAGE C)
     set(SOURCE_FILES ${SOURCE_FILES} ${FILESYSTEM_SOURCES})
     set(HAVE_SDL_FILESYSTEM TRUE)