externals: libusb: Use autotools for MinGW

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.