sdl2_gfx: new Makefile

Added support for SDL2_gfx (based on SDL_gfx which was already there ;))
SDL has historically recommended to include "SDL.h", for the sake of compiler portability I think, which was followed in add-on libraries such as SDL_gfx (cf. their samples).

This is probably outdated and makes little sense in MXE, especially if <SDL.h> is used right above, so I've used <xxx.h> everywhere :)