After updating to 1.0.24, MinGW fails to build libusb as a result of
numerous errors. So we build libusb their way and let them update the
nontrivial stuff.
This only applies to MinGW: the old path is still in use for Linux
toolchains as well as MSVC.
This will dynamically link libusb, since I hit build errors with the old
way we used to resolve the conflict with SDL2.
lat9nq