rpc: Build only when XDR is available
rpc: Build only when XDR is available

Our RPC layer is as tied to XDR as possible. Therefore, if we
haven't detected and XDR library there's not much sense in trying
to build RPC layer.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,jfehlig/libvirt,zippy2/libvirt,zippy2/libvirt,olafhering/libvirt,libvirt/libvirt,libvirt/libvirt,libvirt/libvirt,crobinso/libvirt,olafhering/libvirt,crobinso/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,crobinso/libvirt,jfehlig/libvirt,jfehlig/libvirt