DERBY-2573: Add more logic from 10.10 build script to fix broken builduserdocs target in 10.8; commit derby-2573-17-aa-useAntExec4.diff.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/branches/10.8@1557235 13f79535-47bb-0310-9956-ffa450edef68