Sdl2 shared

And here are some more fixes for a shared build.

I had to disable the MinGW32 build of SDL2, since that doesn't provide DXGI.h (maybe MinGW32 4 has it?) and it was already failing before my changes.

Regarding SDL2_image: I believe keeping <code>--disable-*-shared</code> is the right thing to do, since this only switches between runtime loading and linking against the library directly. We certainly wan the latter behaviour to avoid surprises in the middle of program execution (IMHO).