title: How to write a Makefile rule to download a file only if it is missing?
body: <p>I'm trying to write a Makefile which should download some sources if and only if they are missing.</p>
  
  <p>Something like:</p>
  
  <pre><code>hello: hello.c
      gcc -o hello hello.c
  
  hello.c:
      wget -O hello.c http://example.org/hello.c
  </code></pre>
  
  <p>But of course this causes <code>hello.c</code> to be downloaded every time make command is run. I would like <code>hello.c</code> to be downloaded by this Makefile only if it is missing. Is this possible with GNU make and how to do this if it is?</p>