this gives us a great level of control over how we build SDL2, omitting parts we don't want, and including parts we do want, like libusb support!
windows sdl2 normally doesn't support libusb, but with some magic it can be built and linked in fairly easily, giving windows sdl2 libusb support.
libusb support in sdl2 means official GC adapter support! (resolves#1879)
linux build will be done in a later commit
Morilli