CONFIGURE: Improve detection of xmlrpc_c flags
    
    The pkg-config files for xmlrpc_c libraries are shipped just
    in fedora/rhel due to downstream patch. Debian does not have
    pkg-config files for xmlrpc_c. Therefore we need to fallback to older
    method of detection XMLRPC_*FLAGS which was reverted
    by the commit 1e0143c159134337a00a91d4ae64e614f72da62e
    
    https://fedorahosted.org/freeipa/ticket/6418
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>