How to call multiple builds from a Ant target ?

Just use the ant tag and give the directory names of other build files :

<?xml version="1.0"?>

<project name="bridgedb" default="all" basedir=".">

    <target name="all">

      <ant dir="corelib"/>  

      <ant dir="tools"/>

      <ant dir="makeGdb"/>

      <ant dir="cytoscape-plugin"/>

    </target>

</project>