sdl2_gfx: new Makefile

Added support for SDL2_gfx (based on SDL_gfx which was already there ;))
You don't understand: I don't want to know if an undefined group decided something about MXE (that doesn't motivate me), I'd like to know why I should spend 2-3x more times on shared builds that, I argued, I think are useless, and even harmful due to added complexity and constraints on contributors.

Also usually the issues don't come from the build system, but from proper upstream API declaration and additional packaging tests (I bugged a few upstream devs into this years ago with limited success and I don't wish to do that again) :/