The debian/configure script will only configure binary packages for threads supported by the running kernel and posix. This is limiting in many environments, such as building on a system with no running RT kernel, or building in a chroot where the running RT kernel does not match any kernel in the distro.
The Universal Build has no requirement for a running, RT-enabled kernel. The requirements to build the various flavors are:
POSIX: no requirements (represented in current debian/configure)
PREEMPT_RT: no requirements
Xenomai userland: libxenomai-dev, libxenomai1, maybe xenomai-runtime
Xenomai kthreads: the above, plus linux-headers--xenomai
RTAI kthreads: linux-headers--rtai, others?