title: How to determine the exact lib/include variable generated by find_packge() in config mode?
body: <p>For example, I want to use Eigen3, Pangolin and OpenCV in a cpp project. After some trials, I find the CMakeLists.txt below passes the compilation:</p>
  <pre><code>find_package(Pangolin)
  find_package(Eigen3 REQUIRED)
  
  include_directories(${EIGEN3_INCLUDE_DIRS} ${Pangolin_DIR} ${OpenCV_DIRS)
  
  add_executable(my_proj my_proj.cpp)
  target_link_libraries(my_proj ${Pangolin_LIBRARIES} ${OpenCV_LIBS})
  </code></pre>
  <p>And I find at least ${Eigen3_INCLUDE_DIRS}, ${EIGEN3_DIRS} and ${PANGOLIN_DIR} not working.</p>
  <p>So how to determine the exact variable name given so many possibilities, i.e. all capital or not, with INCLUDE or not , with S or not?</p>