You’re on the right track, use manifestclasspath task. The jarfile attribute is used to create relative links to the jars contained in the fileset.
<manifestclasspath property="jar.classpath" jarfile="${client_work_dir}/HelloWorld.jar">
<classpath>
<fileset name="" dir="${client_work_dir}/lib" includes="*.jar"/>
</classpath>
</manifestclasspath>
<jar jarfile="${client_deploy_dir}/HelloWorld.jar" basedir="${client_work_dir}/compiled">
<manifest>
<attribute name="Main-Class" value="HelloWorld.Main"/>
<attribute name="Class-Path" value=""${jar.classpath}"/>
</manifest>
</jar>