Finally some progress!

I had to add:

{$IFDEF DARWIN}
{$linkframework Cocoa}
{$linkframework SDL2}
{$linkframework OpenGL}
{$linkframework SDL2_image}
{$linkframework SDL2_mixer}
{$ENDIF}

To the main source file to get it to compile. I have no idea why, though. It now crashes when setting up the OpenGL context during runtime, but at least it's compiling now!