sdl2_gfx: new Makefile

Added support for SDL2_gfx (based on SDL_gfx which was already there ;))
Well we decided to support shared build, so we would of course want as many supported shared libs as possible :)

For this library, it looks like the autotools build system is very simple, so you can try simply removing `--host`, `--disable-shared`, and `--prefix` by `$(MXE_CONFIGURE_OPTS)`.