net-libs/libpcap ebuild does not contains libusb in dependencies. But /usr/lib/libpcap.so.1.3.0 depends on library libusb-1.0.so.0.
Aw, configure.in tries to detect system features to determine whether to enable support for USB.
The code in configure.in (lines 1276-1335) still tests for /dev/usbmon which not only requires the devfs to be mounted at that path, but requires CONFIG_USB_MON=[y|m] too. This isn't actually the cause of the libusb dependency, though. Fixed without a revision bump (as IUSE=canusb was added).