Please update ebuilds to require libusbx instead of libusb. A quick grep would imply the following ebuilds:
media-sound/kluppe/kluppe-0.5.4.ebuild media-sound/ardour/ardour-2.8.11.ebuild media-sound/ardour/ardour-3.9999.ebuild (I ran into it building ardour-3.9999) -Reuben
