c-source is not a good relative path

Question

I try to upload a package to hackage and get the brief message

c-sources: C/libqhull_r.c' is not good relative path: same directory segment: ..

I cannot find (with google) any hint what is meant and what I should change (I am not the author of the code, but have permission of the author to upload to hackage).

The cabal file contains

    include-dirs:        C
    C-sources:           C/libqhull_r.c
                       , C/geom_r.c
                       , C/geom2_r.c

and builds with cabal build.