target name init submodules depends init exec executable git failonerr

  <target name="init.submodules" depends="init">
    <exec executable="git" failonerror="true">
      <arg value="submodule"/>
      <arg value="update"/>
      <arg value="--init"/>
    </exec>
  </target>

  <target name="update.submodules" depends="init.submodules">
    <exec executable="git" failonerror="true">
      <arg value="submodule"/>
      <arg value="foreach"/>
      <arg value="(git checkout master; git pull)"/>
    </exec>
  </target>

  <target name="build.submodules" depends="update.submodules">
    <ant antfile="build.xml" dir="submodule1"/>
    <ant antfile="build.xml" dir="submodule2"/>
  </target>