dpkg-cross horrors

So, there's this project for work where I have to provide a full packaged toolchain, including a glibc 2.3.1 which has to be cross-buildable. All of it on stable. So, I installed unstable's toolchain-source on a stable box (seems to work, minus a few silly things which I patched away) and downloaded a glibc_2.3.1-16.dsc and companies from snapshot.debian.net

Then I notice something strange is going on: it doesn't want to cross-build, even though there is code in debian/rules to make that possible. After hours of searching, I find that it does build if I run fakeroot debian/rules clean && debian/rules build manually, but not when I use debuild. Turns out dpkg-cross (which is installed because of toolchain-source) diverts dpkg-buildpackage away, and pollutes the environment if you call it with -asomething.

wouter@rock:~/glibc-2.3.1$ sudo dpkg --purge dpkg-cross
[...]
wouter@rock:~/glibc-2.3.1$ debuild -uc -us -aarm
[builds]

WHY?!?