How to specify relative paths for header files

I tried to specify relative paths to header files in my Nios project. However this does not work.

Only the fully qualified path is ok, e.g., /cygdrive/x/dir1/dir2/project_root/includes

When I use just ./includes it does not work.

The project files are in .../project_root, e.g. .../project_root/.build

What root directory is used for the compiler? how can I change it.

best regards,

jackie