title: Can the C preprocessor detect if a header file cannot be found and issue a specific warning?
body: <p>I have a <strong>C</strong> file that needs a specific header file. If that header file does not exist, I want the preprocessor to issue a specific warning. Something like:</p>
  
  <pre><code>#if !(#include &lt;special.h&gt;)
  #warning "Don't worry, you can fix this."
  #warning "You just need to update this other repo over here:"
  #endif
  </code></pre>
  
  <p>Is this possible with the <strong>C</strong> preprocessor?</p>