Undefined reference to extern variables

Question

I'm using qxmpp library while extending it. I've faced problem when I using extern char* variable from that library. There is a QXmppConstants.h that contains namespaces. I've included qxmpp library it works fine. But when I compiled my project in windows it prompts the following error:

error: undefined reference to ns_bytestreams'

error: undefined reference to ns_disco_info'

error: undefined reference to ns_disco_items'

How to fix that?