/lib/modules/2.6.8-20041123/build/include/linux/modversions.h
This file is missing, which is also (probably) causing the other errors you are seeing. Check where the file modversions.h is (try using locate) and change the path to this file (either edit the makefile, or recompile this file by hand using the gcc line, changing the path to this file).
This file is part of your kernel configuration, so it would probably help to have the kernel installed, and do a
from the kernel-source's top-level directory (usually /usr/src/linux)