title: conditionally import framework
body: <p>well to begin with I'm sure this is a simple question.</p>
  
  <p>I am developing an iPhone app with the iAd Framework, which only runs for iOS 4.0 or higher.</p>
  
  <p>Still, I wanna choose a iPhone OS 3.0 deployment target, which causes everything to crash.</p>
  
  <ul>
  <li>How do I conditionally include the iAd framework?
  ...I mean, it would be something like:
  ...if([[UIDevice currentDevice] systemVersion]>=4.0]) #import </li>
  </ul>
  
  <p>Obviously this won't work because I don't know the correct syntax. Also:</p>
  
  <ul>
  <li>How do I conditionally declare an AdView* variable?</li>
  <li>How do I conditionally handle this AdView* variable in my implementation file.</li>
  </ul>
  
  <p>If you guys could help me, I will be very well impressed.</p>
  
  <p>Thanks</p>