title: fail to compiling elephant-bird with mvn
body: <p>I just can't solve my issue, even it would probably be obvious to you. So can anyone help me ?</p>
  
  <p>The problem occured when I tried to build elephant bird based on it's source code.
  I executed <code>mvn package</code></p>
  
  <p>And it returned:</p>
  
  <pre><code>[ERROR] BUILD ERROR
  [INFO] ------------------------------------------------------------------------
  [INFO] One or more required plugin parameters are invalid/missing for 'protobuf:run'
  </code></pre>
  
  <p>Inside the definition for plugin 'protobuf-maven-plugin' specify the following:
  </p>
  
  <pre><code>&lt;configuration&gt;
    ...
    &lt;inputDirectories&gt;VALUE&lt;/inputDirectories&gt;
  &lt;/configuration&gt;
  </code></pre>
  
  <p>-OR-</p>
  
  <p>on the command line, specify: '-DinputDirectories=VALUE'</p>
  
  <p>I tried to install protobuf, but it still don't work.
  So, does it means I have specify the param?  And how to specify the param?</p>
  
  <p>Again, thanks a lot.</p>