<taskdef name="javac2" classname="com.intellij.ant.Javac2"/>

which has to be edited to make the task work (or include the libs into the ant classpath, but usually I try to avoid that). The build-file would be more flexible if the taskdef referenced a classpath definition that can even be defined externally in a build-file which includes the generated one:

<taskdef name="javac2" classname="com.intellij.ant.Javac2" classpathref="javac2.classpath" />

Optionally, you could even generate the appropriate path definition based on an IDEA-installation:

<path id="javac2.classpath">
  <fileset dir="${idea.home}/lib">
    <include name="javac2.jar" />
    <include name="jdom.jar" />
    <include name="asm.jar" />
    <include name="asm-commons.jar" />
  </fileset>
</path>

This could be commented out, but would help beginners probably more than just naming the required files.