It reminds me very much of OpenSSH, for which we are very much thankful to the OpenBSD project. It too has a focus on just the bare essentials required to run on one OS - OpenBSD in this case - and a 'portable edition' patch for other platforms.
In my own work keeping Samba portable, we have like most large projects, a portability API (libreplace), mostly trying to emulate the C APIs of other OS versions onto Linux APIs where possible. The Kerberos implementation that we work closely with for the AD DC also has libroken, another portability API, and we link against libbsd for strlcpy etc.
I've done a reasonable amount of work in this area recently, and it is certain the portability doesn't come for free. Indeed, we have two major build systems because (in part) we don't want to drop systems that don't have python!