configure: look for xen-{evtchn,gnt}-unix
configure: look for xen-{evtchn,gnt}-unix

Signed-off-by: David Scott <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@recoil.org>

mirage/ocaml-vchan,mirage/ocaml-vchan,djs55/ocaml-vchan,djs55/ocaml-vchan,mirage/ocaml-vchan,djs55/ocaml-vchan