I finally got it:
In processing/core/build.xml
:
Change the last target to:
<target name="build" depends="compile" description="Build core library">
<jar basedir="bin" destfile="library/core.jar">
</jar>
<jar basedir="bin" destfile="library/core-sources.jar">
<fileset dir="src" includes="**/*.java"/>
</jar>
</target>