title: How can I avoid Makefile failure if file doesn't exist?
body: <p>I've got a dependency between a few files, where one is generated from the other. For example <code>bar.txt</code> is needed to generate <code>foo-temp.xml</code> is needed to generate the final <code>foo.xml</code> file. In my Makefile this is expressed as follows:</p>
  
  <pre><code>foo-temp.xml: bar.txt
      do-magic -o foo-temp.xml bar.txt
  
  foo.xml: foo-temp.xml
      do-more-magic -o foo.xml foo-temp.xml
  </code></pre>
  
  <p>As long as all files exist, everything works smoothly. But there are instances where I provide <code>foo-temp.xml</code> manually instead of generating it from <code>bar.txt</code>; in fact, <code>bar.txt</code> doesn't even exist in that case.</p>
  
  <p>How can I express this "non-existence" in my Makefile without having it fail because <code>bar.txt</code> does not exist?</p>