title: How to find in CMAKE file if OpenCV is compiled with CUDA
body: <p>i am using opencv with another library. So i would like to compile a class if OpenCV has CUDA.
  i need to acheive this in Cmake file. But i cannot find any variable that tells whether OpenCV
  has CUDA or not.</p>
  
  <p>FindOpenCV.cmake defines these variables</p>
  
  <p>OpenCV_FOUND
    OpenCV_LIBS
    OpenCV_INCLUDE_DIR
    OpenCV_VERSION</p>
  
  <p>now how can i find out if OpenCV is compiled with CUDA or not from this cmake file?</p>