title: Use Environment Variable in WiX script to set Install location
body: <p>I am creating WiX installer to drop my custom VS 2010 extension inside Visual Studio Extensions folder. I want to use the system environment variable <code>VS100COMNTOOLS</code> to figure out the VS installed path inside WiX script. I would like to use a relative path syntax like <code>%VS100COMNTOOLS%\..\IDE\Extensions</code> to get to the <code>C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\Extensions</code> folder or wherever user chose to install VS 2010.  </p>
  
  <p>Can somebody please suggest me how I achieve this in Wix script?</p>