Using an external header file I have a project that is part of a larger repository, and shares header files with non-Arduino C programs.

I know that it's possible to use these headers by copying them to the libraries folder, or the sketch folder, but I would rather use them with a relative path (which seems to not work), so I can avoid copying files around for every repository clone or update.

Is there a solution that doesn't involve moving/copying the header file?

EDIT:
Although there is no full solution for this question,
I have since found that using the PlatformIO 
IDE instead solves the problem.
You can use header files with an absolute name, like:

#include "c:\Temp\x.h"


Another way that might help is to make the include path like:

#include "q:\x.h"


And use Windows to map driver letter Q to the path you need.