feature #2245: By default compiles with old xmlrpc-c

Now old_xmlrpc scons parameter is changed to new_xmlrpc. By default
it compiles with xmlrpc-c < 1.31. In case you want to compile with
version >= 1.31 you can add new_xmlrpc=yes to scons command and
the resulting binary will have new configuration parameters for
xmlrpc server.

http://dev.opennebula.org/issues/2245