build C++ program with opencv library

如果不是自己 build from source，用 apt install libopencv-dev 就可以裝 distribution 版本的 opencv(4) library
然後因為是 distribution package，用 pkg-config 可以找到：

$ pkg-config --cflags opencv4
-I/usr/include/opencv4/opencv -I/usr/include/opencv4

$ pkg-config --libs opencv4
-lopencv_stitching -lopencv_aruco -lopencv_bgsegm -lopencv_bioinspired -lopencv_ccalib -lopencv_dnn_objdetect -lopencv_dnn_superres -lopencv_dpm 
-lopencv_highgui -lopencv_face -lopencv_freetype -lopencv_fuzzy -lopencv_hdf -lopencv_hfs -lopencv_img_hash -lopencv_line_descriptor -lopencv_quality 
-lopencv_reg -lopencv_rgbd -lopencv_saliency -lopencv_shape -lopencv_stereo -lopencv_structured_light -lopencv_phase_unwrapping -lopencv_superres 
-lopencv_optflow -lopencv_surface_matching -lopencv_tracking -lopencv_datasets -lopencv_text -lopencv_dnn -lopencv_plot -lopencv_ml -lopencv_videostab 
-lopencv_videoio -lopencv_viz -lopencv_ximgproc -lopencv_video -lopencv_xobjdetect -lopencv_objdetect -lopencv_calib3d -lopencv_imgcodecs 
-lopencv_features2d -lopencv_flann -lopencv_xphoto -lopencv_photo -lopencv_imgproc -lopencv_core

所以build 的時候就：

g++ main.cpp `pkg-config --cflags --libs opencv4`

就可以了。