build dist only when releasing

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/jcs/trunk@1636423 13f79535-47bb-0310-9956-ffa450edef68
seems I messed up version, sorry + adding a BUILDING.txt file

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/jcs/trunk@1636425 13f79535-47bb-0310-9956-ffa450edef68