title: How do you enable ARC project-wide in Xcode 4.2
body: <p>I have an iOS app that I want to convert to using <strong>ARC</strong>. I would just use the migration tool, but it errors out for me consistently during the preview phase.</p>
  
  <p>I know I can use the <code>-fobjc-arc</code> compiler directive on a file-by-file basis, but I want to enable <strong>ARC</strong> on the entire project, then turn off individual classes using <code>-fno-objc-arc</code>. </p>
  
  <p>New projects in 4.2 can use ARC by default, so there must be a switch somewhere.</p>
  
  <p>Can anyone help me to convert the project to <code>ARC</code></p>