If the system has libusb installed but USE_SYS_LIBUSB is false, the
static libusb will link against the system libusb (as pkg_check_modules
sets LIBUSB_LIBRARIES, and 3rdparty/libusb_cmake/libusb.cmake links the
usb-1.0-static target with LIBUSB_LIBRARIES). Unset LIBUSB_LIBRARIES to
avoid this when using the libusb submodule.