> Understand that C++ compilers by default have no idea where most things are

C, C++ compilers by default expect libraries to be installed in the correct place, which would mean that you don't need to specify flags at all.

Also nothing stops you from writing:

    #include </home/john/../../opt/my/stupid/path/last-project/final/finalv2/work/symlink-somewhere-else/foo/quick-test/school/c-tutorial/v9/1-introduction/header-file.h>