<path id="build.classpath">
<fileset dir="${basedir}">
<include name="lib/*.jar"/>
</fileset>
</path>
<pathconvert property="manifest.classpath" pathsep=" ">
<path refid="build.classpath"/>
<mapper>
<chainedmapper>
<flattenmapper/>
<globmapper from="*.jar" to="lib/*.jar"/>
</chainedmapper>
</mapper>
</pathconvert>
<target depends="compile" name="buildjar">
<jar jarfile="${basedir}/${test.jar}">
<fileset dir="${build}" />
<manifest>
<attribute name="Main-Class" value="com.mycompany.TestMain"/>
<attribute name="Class-Path" value="${manifest.classpath}"/>
</manifest>
</jar>
</target>
For further information check out this article.