title: Is #pragma once a safe include guard?
body: <p>I've read that there is some compiler optimization when using <code>#pragma once</code> which can result in faster compilation.  I recognize that is non-standard, and thus could pose a cross-platform compatibility issue.</p>
  
  <p>Is this something that is supported by most modern compilers on non-windows platforms (gcc)?</p>
  
  <p>I want to avoid platform compilation issues, but also want to avoid the extra work of fallback guards:</p>
  
  <pre><code>#pragma once
  #ifndef HEADER_H
  #define HEADER_H
  
  ...
  
  #endif // HEADER_H
  </code></pre>
  
  <p>Should I be concerned? Should I expend any further mental energy on this?</p>