Implement a fallback mechanism for OpenCV detection: If FIND_PACKAGE(OpenCV) can not find OpenCV, try to find it again using another version of FindOpenCV.cmake which is shipped with libkface and can still find OpenCV on some systems.

Tested under debian sid:

Normal installation of OpenCV via apt-get: OpenCV is found at the first try.
Remove /usr/share/opencv/OpenCVConfig.cmake: First try fails, but the FindOpenCV.cmake shipped with libkface still finds OpenCV at the second try.

If no OpenCV is installed at all, CMake stops after the second attempt, as it should be.