title: Visual Studio - "Unknown Compiler version" after upgrading
body: <p>I have a Visual Studio 2015 C++ project which I wanted to upgrade to VS 2017. I had already denied the automatic prompts to upgrade my project when I first opened my project in VS 2017 (wasn't sure I wanted to upgrade at the time) so I couldn't update my project that way. Instead, I just went to my project's:</p>
  
  <p>Properties->Configuration Properties->General</p>
  
  <p>And in there I updated the 'Platform Toolset' option to 'Visual Studio 2017 (v141)' and also changed the 'Window SDK Version' to 10.0.16299.0 from windows 8.1. After performing this manual upgrade I now get the warning when building:</p>
  
  <pre><code>Unknown compiler version - please run the configure tests and report the results
  </code></pre>
  
  <p>Why am I getting this warning? I thought by changing the 'Platform Toolset' to VS2017 I would be using the VC2017 compiler. How do I fix this warning?</p>