lib/cmake: check for libusb_handle_events_timeout_completed()

libusb < 1.0.9 doesn't have libusb_handle_events_timeout_completed(),
but libusb <= 1.0.8 doesn't have version.h, so we need to check
for the function.

The cmake-code was borrowed from UHD, which also checks
for libusb_error_name(), we add that as well since it might come
handy later on.

Signed-off-by: Steve Markgraf <steve@steve-m.de>