title: DT_DIR undefined
body: <p>I want to check if file returned by <code>readdir</code> is directory. 
  I tried do it using <code>DT_DIR</code> constant (as <code>man readdir</code> says) but it's undefined. What file should I include to get it?</p>
  
  <p>Now I use</p>
  
  <pre><code>#include &lt;sys/types.h&gt;
  #include &lt;dirent.h&gt;
  #include &lt;stdlib.h&gt;
  #include &lt;errno.h&gt;
  </code></pre>
  
  <p>gcc version is 4.6.1</p>
  
  <p>Compilation string:</p>
  
  <pre><code>gcc a.c --std=c99 -Wall
  </code></pre>