title: how appoint a specific windows sdk version for cmake build?
body: <p>I have vs 2017 and   I can set Platform Toolset in IDE:</p>
  
  <ul>
  <li>Visual Studio 2017 - Windows XP (v141_xp)</li>
  <li>Visual Studio 2017 (v141) </li>
  <li>Visual Studio 2015 (v140)</li>
  <li>Visual Studio 2015 - Windows XP (v140_xp)</li>
  </ul>
  
  <p>I want to cmake build many library(opencv,zlib...) ,and want to build two version:</p>
  
  <ul>
  <li>one for xp(Visual Studio 2017 - Windows XP (v141_xp)),</li>
  <li>one for windows 10(Visual Studio 2017 (v141))</li>
  </ul>
  
  <p>of course I can cmake output *.sln and open sln ,set up Platform Toolset,build.... but I want to use cmake --build . automatic build</p>
  
  <p>How can I appoint platform toolset  for cmake?</p>