title: XCode can't find base SDK
body: <p>XCode 3.2.3 Beta does not allow building for below 3.2. </p>
  
  <p>I opened a project that was made in an earlier XCode. 
  For some reason, the base SDK is reported as missing even after changing it to 4.0.
  Other projects from earlier XCode do not have this issue.</p>
  
  <p>What can I do?</p>