title: Check if generating a Visual Studio solution or Makefile from CMake
body: <p>How can I know in my CMakeLists.txt if I'm generating a Visual Studio solution or a Makefile?</p>
  
  <p>I need to add external projects to the solution like this:</p>
  
  <pre><code>INCLUDE_EXTERNAL_MSPROJECT(cs-tests ${CMAKE_CURRENT_SOURCE_DIR}/cstests/cstests.csproj)
  </code></pre>
  
  <p>But I only want to do it when calling </p>
  
  <pre><code>cmake .. -DCMAKE_BUILD_TYPE=Debug -G "Visual Studio 12 2013"
  </code></pre>
  
  <p>not when I call:</p>
  
  <pre><code>cmake -G "NMake Makefiles" ..
  </code></pre>