Add COMPONENTS support to the OpenCVConfig.cmake (Feature #843)

Hello,

I use OpenCV thank's to cmake... So I do :
find_package(OpenCV REQUIRED)

All works fine. But I have some programs which don't use some modules of
OpenCV, and I would like to be able to do :
find_package(OpenCV COMPONENTS opencv_core opencv_imgproc)
My goal is to minimize the library dependency of my programs.

I think this could be done replacing the following line in the
OpenCVConfig.cmake :
set(OPENCV_LIB_COMPONENTS opencv_core opencv_imgproc opencv_features2d opencv_gpu opencv_calib3d opencv_objdetect opencv_video opencv_highgui opencv_ml opencv_legacy opencv_contrib opencv_flann)

By these lines :

list(LENGTH OpenCV_FIND_COMPONENTS COMPONENTS_COUNT)
if(${COMPONENTS_COUNT} EQUAL 0)
set(OPENCV_LIB_COMPONENTS opencv_core opencv_imgproc opencv_features2d opencv_gpu opencv_calib3d opencv_objdetect
opencv_video opencv_highgui opencv_ml opencv_legacy opencv_contrib opencv_flann)
else(${COMPONENTS_COUNT} EQUAL 0)
set(OPENCV_LIB_COMPONENTS ${OpenCV_FIND_COMPONENTS})
endif(${COMPONENTS_COUNT} EQUAL 0)

I am a CMake newbie so i think that my patch proposition could be enhancend.

Cheers,

A. BARRAL