sdl2_gfx: new Makefile

Added support for SDL2_gfx (based on SDL_gfx which was already there ;))
From personal experience, supporting shared build requires 2-3 more time -- because it's harder to do, and because upstream projects like to break it on a regular basis (dllexport/dllimport/etc.).  Its purpose is also limited AFAIK (people shipping uploaded .dll dependencies will also ship an application rebuild anyway).

I appreciated MXE innovative stand on building static libs only, so for the sake of my motivation, do we have documentation or rationale on supporting shared builds in MXE? :)