Your link line will also need to change to build with the Unix support code. The first thing that will need to change are the start files (crt0.o for a.out and crt*.o for ELF). The Unix support specific start files are installed in the lib/unix subdirectory of your installed OSKit tree.
The second step is to put unix_support.o (unix_support_pthreads.o if you're using pthreads) and the appropriate Native OS library (liblinuxsys.a or libfreebsdsys.a) first in the link line and add -loskit_unix to the list of libraries.
Getting all of this correct is messy and a pain. Look at the changes that were made in the example kernel makefiles (examples/x86/GNUmakerules vs. examples/unix/GNUmakerules).
You can also use the installed GCC driver script (bin/ix86-oskit-gcc). This turns your native compiler into a cross-compiler for the OSKit by causing it to use a different ``specs'' file (the specs file can be found in the OSKit tree at unsupported/scripts/gcc-driver-script-unix.specs). Currently the driver script will only work on an a.out system.