title: Execution of ant file within exec task
body: <p>I want to run my ant i.e. build.xml file in parallel execution along with the ongoing execution of a task.
  I am using Exec task to achieve this. i.e. I am using ant to run the build.xml file within Exec task but facing the following error:
  ERROR: exec doesn't support the nested "ant" element.</p>
  
  <p>My excerpt of the code is:</p>
  
  <pre><code>&lt;if&gt;
      &lt;istrue value="${parallel.exec}" /&gt;
      &lt;then&gt;
  
          &lt;!-- Parallel execution of task --&gt;
          &lt;mkdir dir="${buildroot.dir}/product/${build-log.dir}" /&gt;
          &lt;exec dir="../../apollo" executable="/bin/sh" spawn="true"&gt;
              &lt;ant antfile="${buildroot.dir}/product/abs-build.xml" /&gt;
          &lt;/exec&gt;
      &lt;/then&gt;
  </code></pre>