Build with OpenCV 4.1

This includes the possibility to register 2DTrackables.

EM_ARTK_FLAGS="-msse -msse2 -msse3 -mssse3 -I$OURDIR/depends/emscripten/opencv-4.1 -I$OURDIR/depends/emscripten/opencv-4.1/modules/core/include -I$OURDIR/depends/emscripten/opencv-4.1/modules/highgui/include -I$OURDIR/depends/emscripten/opencv-4.1/modules/imgcodecs/include -I$OURDIR/depends/emscripten/opencv-4.1/modules/videoio/include -I$OURDIR/depends/emscripten/opencv-4.1/modules/imgproc/include -I$OURDIR/depends/emscripten/opencv-4.1/modules/calib3d/include -I$OURDIR/depends/emscripten/opencv-4.1/modules/features2d/include -I$OURDIR/depends/emscripten/opencv-4.1/modules/flann/include -I$OURDIR/depends/emscripten/opencv-4.1/modules/video/include -I$OURDIR/ARX/OCVT/include"

emconfigure cmake .. -DCMAKE_BUILD_TYPE=${DEBUG+Debug}${DEBUG-Release} -DCMAKE_CXX_FLAGS="$EM_FLAGS $EM_ARTK_FLAGS" -DCMAKE_C_FLAGS="$EM_FLAGS $EM_ARTK_FLAGS" -DCMAKE_C_FLAGS_RELEASE="-DNDEBUG -O3" -DCMAKE_CXX_FLAGS_RELEASE="-DNDEBUG -O3" -DCMAKE_EXE_LINKER_FLAGS_RELEASE="-O3"