sdl2_gfx: new Makefile

Added support for SDL2_gfx (based on SDL_gfx which was already there ;))
> shared builds that, I argued, I think are useless, and even harmful due to added complexity and constraints on contributors.

All valid points that I've also encountered, however, windows builds are painful and if we follow that logic we should avoid those also. Wait, what?

> I'd like to know why I should spend 2-3x more times on shared builds

Conceptually, the original contributor is in the best position to enable shared builds due to their familiarity with the package. It is exactly because we've found shared builds are so hard that we're asking if you can help with it up front.

> rationale on supporting shared builds in MXE?

Mostly because many users have asked for it over the years, and also we'd like to combine the efforts of a large fork (http://hg.octave.org/mxe-octave/) that requires shared builds for some technical reasons (possibly qtwebkit).