Generate manifest class-path from <classpath> in Ant

In the build file below, the jar target refers to the jar.class.path property for the manifest class-path. The compile target refers to project.class.path

There is redundancy here, because jar.class.path and project.class.path are very similar. They must be both updated when libraries are added, which can be a pain if the list of libraries gets very long. Is there a better way? Any solution must be cross-platform and always use relative paths.

Edit:
It should generate the JAR classpath from a fileset and not the other way around, so I can use wildcards to e.g. include all JAR files in a directory.

<?xml version="1.0"?>
<project name="Higgins" default="jar" basedir=".">

    <property name="jar.class.path" value="lib/forms-1.2.0.jar lib/BrowserLauncher.jar"/>

    <path id="project.class.path">
      <pathelement location="build"/>
      <fileset dir="lib">
        <include name="forms-1.2.0.jar"/>
        <include name="BrowserLauncher.jar"/>
      </fileset>
    </path>

    <target name="prepare">
        <mkdir dir="build"/>
    </target>

    <target name="compile" depends="prepare" description="Compile core sources">
        <javac srcdir="src"
               includes="**"
               destdir="build"
               debug="true"
               source="1.5">
          <classpath refid="project.class.path"/>
        </javac>
    </target>

    <target name="jar" depends="compile" description="Generates executable jar file">
        <jar jarfile="higgins.jar">
            <manifest>
                <attribute name="Main-Class" value="nl.helixsoft.higgins.Main"/>
                <attribute name="Class-Path" value="${jar.class.path}"/>
            </manifest>
            <fileset dir="build" includes="**/*.class"/>            
            <fileset dir="src" includes="**/*.