Package opencv was not found in the pkg-config search path
From your question I guess you are using Ubuntu (or a derivate). If you use:
apt-file search opencv.pc
then you see that you have to install libopencv-dev.
After you do so, pkg-config --cflags opencv and pkg-config --libs opencv should work as expected.
with opencv 4.0;
- add
-DOPENCV_GENERATE_PKGCONFIG=ONto build arguments -
pkg-config --cflags --libs opencv4instead of opencv