Make libusbx-1.0.15 the only default stable provider for all systems.
Add libusbx-1.0.16-rc3 as an option to test the new upcoming release.
With 1.0.16 libusb and libusbx are merging back together, so the separation doesn't make sense anymore.
The 1.0.16 RCs might give better results on MacOS X, lots of work has gone in the Darwin backend.
Defaults to stable, but just set LIBUSB="beta" in build/common.sh to switch to 1.0.16 RCs.