title: Xcode Won't Compile To Simulator
body: <p>I've just updated Xcode 4.5 via the App Store, from my previous version which I got from my Apple Developer Account. Since the update I can't compile any apps into the Simulator. I can compile onto a device, e.g. my iPhone. </p>
  
  <p>The error I recieve is </p>
  
  <pre><code>    fatal error: file 
      '/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/
  Developer/SDKs/iPhoneSimulator6.0.sdk/usr/include/Availability.h'
           has been modified since the precompiled header was built
  
  1 error generated.
  </code></pre>