title: Visual Studio wants Ninja Generator even though I don't have it installed
body: <p>I am trying to set up a simple project so I can learn some OpenGL. Here is my CmakeList.txt:</p>
  
  <pre><code>cmake_minimum_required (VERSION 3.8)
  
  project ("opengl_1")
  
  set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/modules/")
  
  find_package(OpenGL REQUIRED)
  if (OpenGL_FOUND)
      include_directories(${OPENGL_INCLUDE_DIR})
  endif()
  
  find_package(GLFW REQUIRED)
  if (GLFW_FOUND)
      include_directories(${GLFW_INCLUDE_DIR})
  endif()
  
  file(GLOB_RECURSE SRC "src/*")
  file(GLOB_RECURSE INCLUDES "include/*")
  
  include_directories("include")
  
  # Add source to this project's executable.
  add_executable (opengl1 ${SRC} ${INCLUDES})
  target_link_libraries(opengl1 ${OPENGL_LIBRARIES} ${GLFW_LIBRARIES})
  </code></pre>
  
  <p>I had abandoned Visual Studio Code for Visual Studio 2017 due the problem generating the compile commands file on Windows. It required Ninja generator and I had a ton of trouble getting cmake to use it.</p>
  
  <p>So now, I <code>cd</code> in my project directory and do <code>cmake .</code>:</p>
  
  <pre><code>-- Building for: Visual Studio 15 2017
  -- The C compiler identification is MSVC 19.14.26431.0
  -- The CXX compiler identification is MSVC 19.14.26431.0
  -- Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Tools/MSVC/14.14.26428/bin/Hostx86/x86/cl.exe
  -- Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Tools/MSVC/14.14.26428/bin/Hostx86/x86/cl.exe -- works
  -- Detecting C compiler ABI info
  -- Detecting C compiler ABI info - done
  -- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Tools/MSVC/14.14.26428/bin/Hostx86/x86/cl.exe
  -- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Tools/MSVC/14.14.26428/bin/Hostx86/x86/cl.exe -- works
  -- Detecting CXX compiler ABI info
  -- Detecting CXX compiler ABI info - done
  -- Detecting CXX compile features
  -- Detecting CXX compile features - done
  -- Found OpenGL: opengl32
  -- Found GLFW: C:/libraries/glfw/include (found version "3.2.1")
  -- Configuring done
  -- Generating done
  -- Build files have been written to: ...
  </code></pre>
  
  <p>No mention of Ninja generator anywhere. However, when I open the solution in VS2017 and go to update intellisense with "generate" I get:</p>
  
  <pre><code>1&gt; Does not match the generator used previously: Visual Studio 15 2017
  1&gt; Either remove the CMakeCache.txt file and CMakeFiles directory or choose a different binary directory.
  CMake Error: Error: generator : Ninja
  Does not match the generator used previously: Visual Studio 15 2017
  Either remove the CMakeCache.txt file and CMakeFiles directory or choose a different binary directory.
  </code></pre>
  
  <p>When I remove these things the error still occurs.</p>
  
  <p>What is going on here? If I require Ninja anyway can someone tell me what I need to do?</p>
  
  <p>Thank you!</p>