sdl2_gfx: new Makefile

Added support for SDL2_gfx (based on SDL_gfx which was already there ;))
This sounds cloudy, I think we need documentation on this. For now MXE wants to support shared builds a) because some users want it for unknown reasons and b) because the Octave fork requires it for unknown technical reasons.

I quickly skimmed through my notes and saw that the main reason for shared builds would be to use LGPL libs in proprietary apps with minimum hassle - which is a debatable goal for MXE, but a clear reason nonetheless.

In SDL2_gfx fix, I see that:
- it's zlib-licensed so legal issues with static builds
- it's not used in Octave
- none of the other SDL* package seem to support shared builds
- there no dev documentation on shared builds, and at first glance requires a full rebuild

So I'd suggest committing sdl2_gfx as-is, and wait for a user to provide a good reason, in which case I'll have a look. What do you think?