Add OpenCV_DIR variable to CMake cache also if library not found so user can specify it.
Add OpenCV_DIR variable to CMake cache also if library not found so user can specify it.

sk1712/IRTK,sk1712/IRTK,BioMedIA/IRTK,BioMedIA/IRTK,BioMedIA/irtk-legacy,ghisvail/irtk-legacy,BioMedIA/irtk-legacy,sk1712/IRTK,BioMedIA/IRTK,ghisvail/irtk-legacy,BioMedIA/IRTK,sk1712/IRTK,ghisvail/irtk-legacy,BioMedIA/irtk-legacy,BioMedIA/irtk-legacy,ghisvail/irtk-legacy