Hello, I got further down with this (I happen to be maintainer of the at-spi2-core package :) ), the difference here is that the dbus-x11 package is not installed in the Linux case. If I install the dbus-x11 package, Linux starts getting the same issue: /usr/bin/dbus-launch gets launched to autostart a session dbus bus and libatspi then tries to find its at-spi2-core service, to no avail. Normally this only produces a glib warning, but apparently the tests here enable making glib warnings fatal, thus the trap.
So I was wondering whether it'd be a bug of the hurd port or of libatspi, but apparently no, it's just an unfortunate combinaison of behavior. Installing at-spi2-core or setting NO_AT_BRIDGE=1 should be fine then. I would say it would be safer to do so on non-hurd ports too, in case at some point the dbus packages end up making dbus-launch available by default on non-hurd too. Samuel