Fix precise scrolling delta logic in scrollWheel.

`[event respondsToSelector:@selector(hasPreciseScrollingDeltas:)]` always returns false, so it cannot be used as a test.

Also, the #if-#else-#endif logic was wrong, preventing `_glfwInputScroll()` from being called had the respondsToSelector test returned true. 

Please see [this comment](https://github.com/glfw/glfw/pull/95#issuecomment-21464735) for further details.