message: Add a git submodules sanity check step to the build
  This commit adds a specific error message to the build that tells people
  to restore git submodules if they forgot to read the README.
  was the straw that broke the camel's back.
diff: <EnablePREfast>true</EnablePREfast>
  </ClCompile>
  </ItemDefinitionGroup>
  +
  + <!-- Sanity check: Make sure the user followed the README and initialized git submodules. -->
  + <Target Name="EnsureSubmodulesExist" BeforeTargets="PrepareForBuild">
  + <PropertyGroup>
  + <ErrorText>This project references git submodule(s) that are missing on this computer. Use `git submodule update --init --recursive` to download them. For more information, see https://github.com/microsoft/terminal#building-the-code. </ErrorText>
  + </PropertyGroup>
  + <Error Condition="!Exists('$(SolutionDir)\dep\wil') Or !Exists('$(SolutionDir)\dep\gsl')" Text="$([System.String]::Format('$(ErrorText)'))" />
  + </Target>
  </Project>
  