如果iOS版本是8或更高，只初始化全局变量?

I would like to use SKLightNode in my new game. As you might know iOS 7.1 does not support SKLightNode. I would still like to use it for the ones that are on 8.0 or higher. The users with 7.1 or less will have to play without the lighting effects.I would like to use SKLightNode in my new game.