title: Detect MSVC version in GYP
body: <p>I'm trying to detect a msvc version during <code>node-gyp configure</code> in my binding.gyp file.
  Basically, I want to be able to link against particular 3rdparty library based on Visual C++ version:</p>
  
  <pre><code>['OS=="win"' and 'toolset="vc12"' , {
      'libraries': [
          "opencv/lib/vc12/opencv_world300.lib"
      ],
  }],
  
  ['OS=="win"' and 'toolset="vc11"' , {
      'libraries': [
          "opencv/lib/vc11/opencv_world300.lib"
      ],
  }],
  
  ['OS=="win"' and 'toolset="vc10"' , {
      'libraries': [
          "opencv/lib/vc10/opencv_world300.lib"
      ],
  }]
  </code></pre>
  
  <p>Unfortunately, neither <code>toolset</code>, nor <code>_toolset</code> or even <code>$(TOOLSET)</code> variables are defined in GYP. 
  I wasn't able to find such variable in GYP documentation. Is it possible at all?</p>