Title: Porting to Debian GNU variants: Targetting POSIX instead of GNU/Linux

Debian is or indents to be an universal distribuition, and thus
have multiple kernels underneath

List the technical porting problems like build time failures or
runtime wrong assumptions. What can Debian Developers do to help the
GNU/Hurd port and make their packages and infrastructure friendlier
to such ports and common things to avoid in order to not break
packages on GNU/Hurd. If time allows a porting demo may be done.