title: Cmake override find_package for a given target
body: <p>We have a CMakeLists.txt that links (for instance) opencv to our various binaries. This is done as follow:   </p>
  
  <pre><code>find_package(OpenCV REQUIRED core imgproc highgui contrib) 
  target_link_library(XXX opencv_core)
  </code></pre>
  
  <p>We also would like to allow the person building the library to provide its own opencv library. It seems that this could be done setting -DCMAKE_PREFIX_PATH to the right path.  </p>
  
  <pre><code>cmake -DCMAKE_PREFIX_PATH=".../mybuild/include;.../mybuild/lib" .  
  </code></pre>
  
  <p>However I would like to be sure the library used is exactly the one specified by the client (i.e. if there is nothing in /mybuild/lib the configuration fails).  </p>
  
  <p>How can I allow somebody building the library to override the library used ? (if nothing is specified it should fall back to find_package-s)</p>