opentracker: add IPv6 build variant

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Update README
hamlib: avoid build failures with spurious libraries

* always use libusb >= 1.0 (avoid pulling libusb-compat)
* never use libxml2

Signed-off-by: Steven Barth <steven@midlink.org>
hamlib: disable RPC backends to fix some more build breakages

Signed-off-by: Steven Barth <steven@midlink.org>