undefined reference to `facedetect_cnn(unsigned char*, unsigned char*, int, int, int)`

Tried compiling all the examples, but all failed:

>g++ -o detect-camera detect-camera.cpp \
    -I../include -I/usr/include/opencv4 \
    -I/home/maifeevm25apr/libfacedetection/build/install/include \
    `pkg-config --cflags --libs opencv4`
/usr/bin/ld: /tmp/ccs9GyTs.o: in function `main':
detect-camera.cpp:(.text+0x229): undefined reference to `facedetect_cnn(unsigned char*, unsigned char*, int, int, int)'
collect2: error: ld returned 1 exit status

>g++ -o detect-camera detect-image.cpp \
    -I../include -I/usr/include/opencv4 \
    -I/home/maifeevm25apr/libfacedetection/build/install/include \
    `pkg-config --cflags --libs opencv4`
/usr/bin/ld: /tmp/ccv0yeWy.o: in function `main':
detect-image.cpp:(.text+0x1e1): undefined reference to `facedetect_cnn(unsigned char*, unsigned char*, int, int, int)'
collect2: error: ld returned 1 exit status

>g++ -o detect-camera benchmark.cpp \
    -I../include -I/usr/include/opencv4 \
    -I/home/maifeevm25apr/libfacedetection/build/install/include \
    `pkg-config --cflags --libs opencv4`
/usr/bin/ld: /tmp/ccV7X4TP.o: in function `main':
benchmark.cpp:(.text+0x257): undefined reference to `facedetect_cnn(unsigned char*, unsigned char*, int, int, int)'
/usr/bin/ld: benchmark.cpp:(.text+0x2ec): undefined reference to `facedetect_cnn(unsigned char*, unsigned char*, int, int, int)'
collect2: error: ld returned 1 exit status

It keeps giving me facedetect_cnn(unsigned char*, unsigned char*, int, int, int)

Checked out in fb0c773e1fbe30479e5e7c32888de41b9e818b4d

Start with the compile commands and the example entry points detect-camera.cpp, detect-image.cpp, and benchmark.cpp, then inspect the repository's build or installation instructions and libraries for the missing symbol. Reproduce the linker failure at commit fb0c773e1fbe30479e5e7c32888de41b9e818b4d; done means the examples compile and link successfully.