spec: make client-only testing easier

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>

libvirt.spec.in

@@ -15,8 +15,9 @@
 
 # A client only build will create a libvirt.so only containing
 # the generic RPC driver, and test driver and no libvirtd
-# Default to a full server + client build
-%define client_only        0
+# Default to a full server + client build, but with the possibility
+# of a command-line or ~/.rpmmacros override for client-only.
+%{!?client_only:%define client_only 0}
 
 # Now turn off server build in certain cases