I think that's a bug in the libxmmsclient++-dev package. The dependency on boost comes from the xmms2 c++ bindings, so xmms2-client-cpp.pc should contain the corresponding -l statement.
Current upstream xmms2 dropped the runtime dependency on boost. So this bug will be fixed by the next xmms2 version -- To UNSUBSCRIBE, email to [email protected] with a subject of "unsubscribe". Trouble? Contact [email protected]

