title: How to #import <NSObjCRuntime.h> to use objc_msgSend
body: <p>I'd like to import runtime's header to use objc_msgSend but I'm getting:</p>
  
  <pre><code>error: NSObjCRuntime.h: No such file or directory
  </code></pre>
  
  <p>Should I add something to the header search path?</p>