sdl2_gfx: new Makefile

Some modifications made by me (Timothy):
- test program name changed to follow conventions
- checksum fixed

Signed-off-by: Timothy Gu <timothygu99@gmail.com
sdl_gfx, sdl2_gfx: consistently use square brackets for #include
sdl2_gfx: disable on i686-pc-mingw32

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
sdl2_mixer: make configure flag order saner
sdl_gfx: enable shared builds
sdl2_gfx: enable shared build
Merge branch 'master' of https://github.com/mxe/mxe