Put conditional around gxpc taskdef on third-party

git-svn-id: https://svn.apache.org/repos/asf/incubator/wave/branches/wave-0.4-release@1517930 13f79535-47bb-0310-9956-ffa450edef68
Finish work on third-party dependency sourcing.

This also makes third_party/ a temporary folder than you make yourself.

git-svn-id: https://svn.apache.org/repos/asf/incubator/wave/branches/wave-0.4-release@1517931 13f79535-47bb-0310-9956-ffa450edef68