Fix include of OpenGL headers on OS X systems

On OS X systems, the OpenGL headers are located in an `OpenGl` folder instead of the `GL` folder they're located in in most other systems. See [the official OpenGL intro](https://developer.apple.com/library/mac/documentation/GraphicsImaging/Conceptual/OpenGL-MacProgGuide/opengl_drawing/opengl_drawing.html#//apple_ref/doc/uid/TP40001987-CH404-SW7).

This patch fixes the includes using a preprocessor if-else construct:
```c
#ifdef __APPLE__
#include <OpenGL/gl.h>
#else
#include <GL/gl.h>
#endif
```

I would be glad to discuss this patch, because there are some decisions to be made:
* Should the two `#include` lines be indented?
* Should a different approach be made instead of preprocessor checks (e.g. using cmake)?