DERBY-6071; trunk javadoc build fails with Javadoc failed: java.io.IOException: Cannot run program \bin\javadoc.exe": CreateProcess error=206, The filename or extension is too long.
  adding useexternalfiles=true to the offending javadoc section


git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1445994 13f79535-47bb-0310-9956-ffa450edef68