title: fatal error: limits.h: No such file or directory
body: <p>On OSX, I can't compile simple programs from the command-line when I use an externally-provided compiler (e.g. gcc, or a custom clang install).  System headers like <code>limits.h</code> cannot be found.</p>
  
  <p>Example error messages:</p>
  
  <pre><code>fatal error: limits.h: No such file or directory
  </code></pre>
  
  <p>Or:</p>
  
  <pre><code>fatal error: stdio.h: No such file or directory
  </code></pre>
  
  <p>Or:</p>
  
  <pre><code>fatal error: 'stdlib.h' file not found
  </code></pre>
  
  <p>and so on.</p>
  
  <p>What's wrong?</p>