title: xcode-select active developer directory error
body: <p>Saw the following error when running an <code>npm install</code> which required <code>node-gyp</code>... but could be triggered by anything which requires <code>xcode-select</code>.</p>
  
  <blockquote>
    <p>xcode-select: error: tool 'xcodebuild' requires Xcode, but active
    developer directory '/Library/Developer/CommandLineTools' is a command
    line tools instance</p>
  </blockquote>
  
  <p>What is the problem?</p>