I contacted the buildd admin first, without any answer. Then asked for a
porter build on debian-arm without much more success, though at least,
the message got answers. Note that it's not the first time xulrunner
fails in such a manner. It's just that the buildd is too slow and
doesn't wait long enough for the link to finish.
After thinking about it, i wonder if i've not seen the timeout thing on
another package... Anyways, I'm not surprised building a huge .so file
on a buildd with 60MB RAM (iirc) is pretty slow. It takes 500MB+ on my
computer when I build it...