Several recent patches cleaned up 'make rpm' for the situation
when client_only is true; these were done by manual spec file
editing (since it's relatively hard to come by a RHEL 5 s390
box).  Make it easier to do in the future via a simpler command
line override.

* libvirt.spec.in (client_only): Allow for override.
Signed-off-by: NEric Blake <eblake@redhat.com>