Build a sort-of-systemd-dependent package on kfreebsd

[Alec Leamas <leamas.alec@gmail.com>]

Dear list,

We are about to push the new lirc to stable. As-is, the package declares 
a dependency on systemd and thus rightfully fails to build on kfreebsd 
platforms. This is a pity since the core software lirc builds fine at 
least on FreeBSD 10.3.

However, lirc contains all sorts of systemd stuff: tweaks in 
rules/dh_auto_install, systemd configuration files, one or two tools. 
The configure script copes with all this and builds a different set of 
files on freebsd.

I'm now trying to wrap my head around how to conditionalize a packet 
such as lirc. I'm coming from Fedora/RPM and used to just spread some
%ifarch in the spec file. Now, is something similar possible in debian?

In particular:
  - How can I handle that kfreebsd should install a different set of files?
  - Is it possible to conditionalize the rules file w r t platform?
  - If so, how?


Cheers!

--alec