title: -Wmissing-include-dirs & automatically included header directories
body: <p>When building a project in Xcode (3.1.2), 2 folders are automatically added as header directories with the '-I' option:  </p>
  
  <pre><code>/ (ProjPath) / (ProjName) / build / (BuildConfigName) / include  
  / (ProjPath) / (ProjName) / build / (ProjName).build / (BuildConfigName) / (ProjName).build / DerivedSources
  </code></pre>
  
  <p>I realized this when trying to add <code>-Wmissing-include-dirs</code> to the warnings list in my project.
  A warning is emitted for both folders. This happens for both Cocoa Applications and C++ Dynamic Libraries. I haven't tried any other project type, but I suppose this behavior applies to any project type.</p>
  
  <ol>
  <li>How are these folders used by Xcode?</li>
  <li>Can they be either avoided, or automatically created by Xcode when build begins?</li>
  <li>If not, how to successfully use <code>-Wmissing-include-dirs</code> with Xcode?</li>
  </ol>
  
  <p>Thanks.</p>