I'm hitting this too. I can't hard-code /usr/share/java/ant-contrib.jar in my
build.xml because the project also needs to be built on non-Debian machines
(that put ant-contrib.jar elsewhere) too.
I'm working around this by bundling the ant-contrib jar in my project so that I
can reference it with a build.xml-relative path.
This is the maintainer address of Debian's Java team
debian-j...@lists.debian.org for discussions and questions.