Working solution:
<target name="compile.ane">
<exec executable="${FLEX_HOME}/bin/adt.bat">
<arg value="-package"/>
<arg value="-target"/>
<arg value="ane"/>
<arg value="commExtension.ane"/>
<arg value="extension.xml"/>
<arg line="-swc"/>
<arg line="commExtLib.swc"/>
<arg line="-platform Android-ARM -C android/ ."/>
</exec>
</target>