title: Availability.h, UIKit.h, etc not found
body: <p>I'm having a great problem here.</p>
  
  <p>I recently downloaded Xcode 4.3 from the Mac App Store, because it had the full-screen feature for Lion. And then, my project stopped compiling. It says:</p>
  
  <p>Lexical or Preprocessor Issue <code>Availability.h</code> file not found.</p>
  
  <pre><code>//
  // Prefix header for all source files of the 'TRGame' target in the 'TRGame' project
  //
  
  #import &lt;Availability.h&gt;
  
  #ifndef __IPHONE_3_0
  #warning "This project uses features only available in iPhone SDK 3.0 and later."
  #endif
  
  
  #ifdef __OBJC__
      #import &lt;UIKit/UIKit.h&gt;
      #import &lt;Foundation/Foundation.h&gt;
  #endif
  </code></pre>
  
  <p>That's in the <code>xxxx-Prefix.pch</code> file.</p>
  
  <p>If I comment out the <code>#import Availability.h</code>, it will tell me that it can't find the <code>UIKit.h</code>, and so on...</p>
  
  <p>Help will be MUCH appreciated!</p>