ant plugin only looks for jar files in directory named "target"

I have the following build.xml for use with the ant plugin in snapcraft :

<project name="HelloWorld" basedir="." default="main">

    <property name="src.dir" value="src"/>

    <property name="build.dir" value="build"/>
    <property name="classes.dir" value="${build.dir}/classes"/>
    <property name="jar.dir" value="lib"/>

    <property name="main-class" value="oata.HelloWorld"/>

    <target name="clean">
        <delete dir="${build.dir}"/>
    </target>

    <target name="compile">
        <mkdir dir="${classes.dir}"/>
        <javac srcdir="${src.dir}" destdir="${classes.dir}"/>
    </target>

    <target name="jar" depends="compile">
        <mkdir dir="${jar.dir}"/>
        <jar destfile="${jar.dir}/${ant.project.name}.jar" basedir="${classes.dir}">
            <manifest>
                <attribute name="Main-Class" value="${main-class}"/>
            </manifest>
        </jar>
    </target>

    <target name="run" depends="jar">
        <java jar="${jar.dir}/${ant.project.name}.jar" fork="true"/>
    </target>

    <target name="clean-build" depends="clean,jar"/>

    <target name="main" depends="clean,run"/>

</project>

After building the part which uses the ant plugin I get the error :
"Failed doing build for hello: could not find any built jar files for part".

If I change the line :
 <property name="jar.dir" value="lib"/>
to
 <property name="jar.dir" value="target"/>
the error goes away.

It would be useful to have a way to configure the location the ant plugin looks in for jar files.