title: Conditionally adding build extension
body: <p>Is there a way to conditionally run build plugin extensions. 
  More specifically : </p>
  
  <pre><code>           &lt;extensions&gt;
                  &lt;extension&gt;
                      &lt;groupId&gt;co.leantechniques&lt;/groupId&gt;
                      &lt;artifactId&gt;maven-buildtime-extension&lt;/artifactId&gt;
                      &lt;version&gt;1.0.1&lt;/version&gt;
                  &lt;/extension&gt;
              &lt;/extensions&gt;
  </code></pre>
  
  <p>Apparently you cannot add extension in a profile for eaxample :</p>
  
  <pre><code>&lt;profiles&gt;
      &lt;profile&gt;
          &lt;id&gt;someid&lt;/id&gt;
          &lt;build&gt;
              EXTENSION
          &lt;/build&gt;
      &lt;/profile&gt;
  &lt;/profiles&gt; 
  </code></pre>
  
  <p>any other good solution?</p>