QPID-5304: Allow c++ unit tests to run if XML_LIB env var not set
(The tests for the xml exchange will obviously fail, but at least
you can run anything else you might have been interested in)

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1539505 13f79535-47bb-0310-9956-ffa450edef68