DERBY-2573: Incorporate more logic from the 10.10 build script to fix problem with running builduserdocs target on non-windows systems; checkin  derby-2573-16-aa-useAntExec3.diff.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/branches/10.8@1557234 13f79535-47bb-0310-9956-ffa450edef68