As described in
DERBY-6898, the docs build currently requires that
you set a custom CLASSPATH when running Ant, otherwise you get
We don't have this requirement with the main code build, so I wonder
if we can improve the docs build.xml so that we don't need to remember
to set a custom CLASSPATH prior to runing the docs build.
See the derby-dev email thread from July, 2016, entitled "Need dita help"
for more discussion about this issue.