title: Setting CFLAGS for pr_debug and printk
body: <p>I am trying to understand a Linux kernel module and would like to see the output of <code>pr_debug</code> and <code>printk</code>. I am using GNU Make.<br>
  I understand that to get <a href="http://www.kernel.org/doc/local/pr_debug.txt" rel="nofollow noreferrer">pr_debug</a> messages, we have to use <code>DDEBUG</code>.     </p>
  
  <p>So, how do I enable <code>printk</code> statements  ?</p>
  
  <p>Lets say filename is <code>kvm.c</code>. What is the difference between these two:</p>
  
  <pre><code>      CFLAGS_kvm.o := -DDEBUG
        CFLAGS_kvm.o += -DDEBUG
  </code></pre>
  
  <p>What does this statement do:</p>
  
  <pre><code>      CFLAGS_kvm.o := -I.
  </code></pre>
  
  <p>[Edit]:<br>
   It looks like my usage of square brackets has caused some confusion. Actually by [filename], I meant some file, say kvm.c.</p>