[CentOS] OT: General Help in chroot build environment

I am trying to compile a module for a Realtek 8168 for thinstation.
I have pulled the developer tarball and executed the script to enter its build environment.
I also have the source for the module I want to compile, but when I execute #make clean modules
it errors out. Reading the errors, I can see it tried to look for the kernel source of the systems actual
running kernel. From within the chroot build environment, I run a #uname -r and see it displays exactly
that, and not the build environments expected 2.6.21.1 that it has in the /source/kernel-2.6.21.1/linux-2.6.21.1
dir. I have ran an #export KERNELSRC=/source/kernel-2.6.21.1/linux-2.6.21.1

Any ideas on what's left to convince make it is running with the expected kernel and not my actual one?

Thanks!
jlc