External modules known to fail to build:

bcm5700-source
drbd8-source
exmap-modules-source
fglrx-source
linux-wlan-ng
open-vm-source
realtime-lsm-source
sl-modem-source
squashfs-source
sysprof-module-source
zaptel-source

Todo: grml-kerneladdons and truecrypt.

We have to investigate on that. And we should talk about the state of kvm in-
kernel vs. out-of-tree module.