Doesn't see optional.jar

[Gary Lawson]

We're trying to use the junit optional task, using the following section in
build.xml

<junit>
      <classpath refid="project.classpath" />
      <formatter type="plain" />
      <test name="com.sferics.campfiretest.AllTests" haltonfailure="no" />
</junit>

optional.jar is in /ant/lib and is referenced on the CLASSPATH (belts and
braces).

Despite this, we still get the following error:

/home/gary/build.xml:87: Could not create task of type: junit because I
can't find it in the list of task class definitions.  Common solutions are:
1 execute bin/bootstrap. 2 use taskdef to declare your task. 3 add the task
to defaults.properties.

Any ideas?
TIA
Gary