Could not find module FindOpenCV.cmake

This link discusses this issue while building the project with cmake - 
https://stackoverflow.com/questions/8711109/could-not-find-module-findopencv-cmake-error-in-configuration-process

In my case, the problem was that I was setting the wrong environment variable, which is case-sensitive. Set

OpenCV_DIR, not OPENCV_DIR

Then cmake is able to find opencv.