This program was written for the linux-2.6.23.1 kernel, which should include its own makefile.
First thing's first, to make my modifications to the kernel, these files must go in their appropriate place:
unistd.h -> linux-2.6.23.1/include/asm/unistd.h
prodcons.h -> linux-2.6.23.1/include/linux/prodcons.h
syscalls.h -> linux-2.6.23.1/include/linux/syscalls.h
syscall_table.S -> linux-2.6.23.1/arch/i386/kernel/syscall_table.S
Then, you can compile the kernel and compile prodcons.c to see the producer-consumer problem being solved successfully.