Building an out-of-tree Linux kernel module in a separate object directory

I'm confronting the Linux kernel build system (Kbuild, kernel ≥2.6.28) with the directory structure and build system for a larger project. Our project contains an out-of-tree Linux kernel module, and our directory structure looks like this (simplified, obviously):

checkout/src/common/*.c           source files (common to Linux and other platforms)
checkout/src/linux-driver/*.c     source files (for the Linux kernel driver)
checkout/build/linux/Kbuild       Kbuild
tmp/linux-2.6.xx/                 where the Linux kernel is unpacked and configured
output/linux-arm-debug/           where object files must end up

The build process must not modify anything under checkout, and building the module must not modify anything under tmp/linux-2.6.xx. All output files must end up under output/linux-arm-debug (or whatever architecture and debug variant was selected at build time).

I've read kbuild/modules.txt, and started to write my Kbuild file:

MOD_OUTPUT_DIR = ../../../output/linux-$(ARCH)-$(DEBUG)
obj-m += $(MOD_OUTPUT_DIR)/foo_mod.o
$(MOD_OUTPUT_DIR)/our_module-objs := $(MOD_OUTPUT_DIR)/foo_common.o $(MOD_OUTPUT_DIR)/foo_linux.o

This handles storing the object files in a different directory from where Kbuild lives. Now how can I specify that foo_common.o needs to be compiled from …/checkout/src/common/foo_common.c and foo_linux.o from …/checkout/src/linux-driver/foo_linux.c?