<property name="lib" value="../lib"/>
<property name="outputdir" value="/tmp"/>
...

<path id="myclasspath">
  <pathelement path=".;${lib}/log4j.jar"/>
  <fileset dir="${lib}">
    <include name="**/*.jar"/>
  </fileset>
</path>
...
  <javac srcdir="./src" destdir="${outputdir}/classes" >
    <classpath refid="myclasspath"/>
  </javac>
