Upgrade sdl2 2.0.1

    Upgrade sdl2 branch to 2.0.1 (see note about OS X)
    
    The sdl2 branch did not match upstream's SDL 2.0.0 release:
    
    * SDL_config_nintendods.h and SDL_input.h were not part of the release
    * The binaries did not match the release archives
    * It removed a check for OS X 10.6
    
    For 2.0.1, this branch uses a script in code/SDL2 to perform the upgrade
    with the official binaries.  It does not include libSDL2main.a for OS X
    because the SDL team did not release it.

Could someone create libSDL2main.a for OS X before pulling this?