Your path is wrong. It is not /lib/modules/$(shell uname -r)/build. In my system, what my wrote down is /usr/src/linux-obj/i586/debug. It works. Of course, you should make sure the kernel source and kernel objs are installed.
My Makefile is as fellow: (openSUSE 11.1)
Makefile
obj-m := new.o
all:
make -C /usr/src/linux-obj/i586/debug M=pwd modules
clean:
make -C /usr/src/linux-obj/i586/debug M=pwd clean