title: what is .d file after building with make
body: <p>Sometimes, I find <code>.d</code> files for a given source file.
  For instance, if I compile <code>test.c</code>, I've got</p>
  <pre><code>test.d, test.o
  </code></pre>
  <p>I understand that <code>test.o</code> is the object file but have no idea what is <code>test.d</code> for.
  Could you give any hints or pointers?</p>