The check for build/include_subdir only checks that a subdirectory is present at all; it does not check that it's complete or correct, and it does not allow for top level headers that simply don't have a subdirectory. E.g. if the file structure was

src
|
|-- main.cpp
|
|-- utils.hpp

and we had a main.cpp like

#include "utils.hpp"

int main() {
}

this would raise an error, even though everything is correct.