Don't try to copy the javadoc if generation was skipped

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@1585973 13f79535-47bb-0310-9956-ffa450edef68
detect Java9

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@1585981 13f79535-47bb-0310-9956-ffa450edef68