title: CMake generation fails on VS 2019 C# Project
body: <p>Using VS 2019, my CMake CSharp project complains:</p>
  
  <pre class="lang-none prettyprint-override"><code>CMAKE_CSharp_COMPILER not set, after EnableLanguage
  </code></pre>
  
  <p>and </p>
  
  <pre class="lang-none prettyprint-override"><code>C# is currently only supported for Microsoft Visual Studio 2010 and later
  </code></pre>
  
  <p>after generation. The following files have been used:</p>
  
  <p>CMakeLists.txt:</p>
  
  <pre><code>cmake_minimum_required(VERSION 3.1)
  project(test_proj CSharp)
  
  include(CSharpUtilities)
  set(CMAKE_CSharp_FLAGS "/langversion:latest")
  
  add_executable("test"
      main.cs
  )
  set_target_properties("test" PROPERTIES
      LINKER_LANGUAGE CSharp
      VS_DOTNET_TARGET_FRAMEWORK_VERSION "v4.5"
      VS_DOTNET_REFERENCES
          "Microsoft.CSharp;
          System;
          System.Core;
          System.Data;
          System.Data.DataSetExtensions"
  )
  </code></pre>
  
  <p>CMakeSettings.json:</p>
  
  <pre><code>{
    "configurations": [
      {
        "name": "x64-Release",
        "generator": "Ninja",
        "configurationType": "Release",
        "buildRoot": "${env.USERPROFILE}\\CMakeBuilds\\${workspaceHash}\\build\\${name}",
        "installRoot": "${env.USERPROFILE}\\CMakeBuilds\\${workspaceHash}\\install\\${name}",
        "cmakeCommandArgs": "",
        "buildCommandArgs": "-v",
        "ctestCommandArgs": "",
        "inheritEnvironments": [ "msvc_x64_x64" ],
        "variables": []
      }
    ]
  }
  </code></pre>
  
  <p>It seems that the problem may related to <code>Ninja</code> generator.</p>