title: How to conditional set up a Makefile variable by testing if a file exists
body: <p>For example:
  I want:</p>
  
  <p>if file1 exists:</p>
  
  <pre><code>CLEAN_SRC = *.h file3
  </code></pre>
  
  <p>else</p>
  
  <pre><code>CLEAN_SRC = 
  </code></pre>