add metal support as optionnal renderer in ios App

I'm working on an iOS App with openGL ES rendering. we are working on metal support and we are experiencing a crash at app launch on device with ios 7.1 (we target ios 7.1+ devices)

dyld: Library not loaded: /System/Library/Frameworks/Metal.framework/Metal

looks like even if the Metal.framework is still unused in our app, the app tries to load it and crashes as the framework in not a part on ios7.

How can we dynamically link the metal framework only in case device is ios8+ ?