I downloaded the 2.6.18 kernel and did the following

make defconfig ARCH=um
make menuconfig ARCH=um
make ARCH=um

I got the following error
$make ARCH=um
SYMLINK arch/um/include/kern_constants.h
  SYMLINK arch/um/include/sysdep
  CHK     arch/um/include/uml-config.h
  UPD     arch/um/include/uml-config.h
  CC      arch/um/sys-i386/user-offsets.s
arch/um/sys-i386/user-offsets.c: In function 'foo':
arch/um/sys-i386/user-offsets.c:19: warning: implicit declaration of function 'offsetof'
arch/um/sys-i386/user-offsets.c:19: error: expected expression before 'struct'

The offsetof macro is defined in <linux/stddef.h> but somehow it is not in the include path.

Can anyone help?