title: cmake target_include_directories called with invalid arguments
body: <p>I am preaty new to cmake . I was using makefiles before but due to QtCreator I am forced to use cmake. I am trying to learn glfw as well too. I have following cmake file:-</p>
  
  <pre><code>cmake_minimum_required(VERSION 3.10)
  
  project(untitled)
  find_package(glfw3 3.2 REQUIRED)
  find_package(OpenGL REQUIRED)
  add_executable(${PROJECT_NAME} "main.cpp")
  
  target_include_directories(untitled ${OPENGL_INCLUDE_DIR})
  target_link_libraries(untitled ${OPENGL_gl_LIBRARY})
  </code></pre>
  
  <p>And I get following error:-</p>
  
  <pre><code>CMakeLists.txt:8: error: target_include_directories called with invalid arguments
  </code></pre>
  
  <p>I have no Idea what does it mean. Please help me</p>