Log Message:
Import buildlink3.mk file for devel/libusb-compat

libusb-compat is an API wrapper, emulating devel/libusb through the newer
code from devel/libusb1. On NetBSD, this has multiple advantages, such as
not requiring root privileges when accessing ugen(4) USB devices. We should
therefore consider to default to devel/libusb-compat instead of
devel/libusb on NetBSD at the very least.

Tested with devel/libftdi on NetBSD/amd64.