### Problem description
Commit 1249eb3261 added `#include
<GL/gl.h>` in a file, but MacOS uses `<OpenGL/gl.h>`.
### Implementation description
If compiling on MacOS, use `<OpenGL/gl.h>`. Otherwise, use `<GL/gl.h>`.
### Additional things
There is a redefine, so I used the preprocessor to ignore it for MacOS.
This feels a little awkward to me (but I don't know how to do it
better).
I'm not particularly familiar with C++, so let me know if this is the
right solution. This is also my first pull request, so feel free to
roast me for anything else :)
