title: msbuild 15 failing with missing targeting framework
body: <p>I installed build tools for visual studio 2017 (msbuild 15) and I'm having problems building a solution.  I installed the framework (4.6.1) prior to installing the build tools, so it is present.</p>
  
  <pre><code>C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets(1122,5): error MSB3644: 
  The reference assemblies for framework ".NETFramework,Version=v4.6.1" were not found. 
  To resolve this, install the SDK or Targeting Pack for this framework version or 
  retarget your application to a version of the framework for which you have the SDK or 
  Targeting Pack installed. Note that assemblies will be resolved from the Global Assembly 
  Cache (GAC) and will be used in place of reference assemblies. Therefore your assembly 
  may not be correctly targeted for the framework you intend.
  </code></pre>
  
  <p>EDIT:  Note that in this case, I'm actually trying to do a silent install of msbuild; so I don't have access to the UI. </p>