title: cmake: check if file exists at build time rather than during cmake configuration
body: <p>I have a custom build command that needs to check if a certain file exists. I tried using</p>
  
  <pre><code>IF(EXISTS "/the/file")
  ...
  ELSE()
  ...
  ENDIF()
  </code></pre>
  
  <p>but that test is only evaluated one; when cmake is first run. I need it to perform the test every time a make is done. What's the method to check at make-time? Thanks.</p>