title: gyp - how to specify link library flavor
body: <p>I'm writing a native node module and I'd like to be able to build it in release and debug versions.</p>
  
  <p>The node module links to another library, which has a debug and release version in two different directories.</p>
  
  <p>Now this is where I'm stuck - how do I specify the library directory for the current configuration being built?</p>
  
  <p>I've tried setting that in configurations.debug.link_settings key, but I get the error:
  'link_settings not allowed in the Debug configuration, found in target binding.gyp</p>