OpenCV installation on Windows, missing vc15 folder

However, I set …\opencv3\build\install\ as value for OPENCV_DIR, and still nothing works. I’m compiling a sample project provided by my professor, saved as .cpp, that has only the directive:

#include <opencv2\highgui.hpp>

by using cmd:

g++ -o [file] lopencv_highgui -lopencv_core

and the result is always “no such file or directory bla bla bla”… but inside \build\install\include\opencv2 I can find the library highgui.hpp, indeed.
I tried also to modify the OPENCV_DIR variable according to the path were the .hpp file is, but nothing changes