title: Use NSURLIsExcludedFromBackupKey without crashing on iOS 5.0
body: <p>The check for availability seems to be working fine but I can't seem to set the <code>NSURLIsExcludedFromBackupKey</code> key without getting this crash on launch:</p>
  
  <blockquote>
    <p>dyld: Symbol not found: _NSURLIsExcludedFromBackupKey Referenced from:
    /Users/sam/Library/Application Support/iPhone
    Simulator/5.0/Applications/B0872A19-3230-481C-B5CE-D4BDE264FBDF/Transit.app/Transit
    Expected in:
    /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Foundation
    in /Users/sam/Library/Application Support/iPhone
    Simulator/5.0/Applications/B0872A19-3230-481C-B5CE-D4BDE264FBDF/Transit.app/Transit</p>
  </blockquote>
  
  <p>Here's my method:</p>
  
  <pre><code>- (BOOL)addSkipBackupAttributeToItemAtURL:(NSURL *)URL {    
      if (&amp;NSURLIsExcludedFromBackupKey == nil)
          return NO;
  
      NSError *error;
      [URL setResourceValue:[NSNumber numberWithBool:YES] forKey:NSURLIsExcludedFromBackupKey error:&amp;error];
      return (error != nil);
  }
  </code></pre>
  
  <p>Crash goes away if I comment out this line:</p>
  
  <pre><code>[URL setResourceValue:[NSNumber numberWithBool:YES] forKey:NSURLIsExcludedFromBackupKey error:&amp;error];
  </code></pre>
  
  <p>Do I have to weak-link Foundation?</p>
  
  <p>EDIT: not sure if it makes a difference, but this method is put inside an <code>NSFileManager</code> category.</p>