Glad you solved it though I think my patch solution is more organic.
Also, nftables has a hardcoded XML schema path that needs changing if one wants to build the PDF documentation on Slackware:
@@ -1,6 +1,6 @@
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
Slackware uses LFS path conventions for docbook as does Arch Linux. Debian, OpenSUSE, and others use the path nftables used.
If drmozes happens on this thread maybe he can shed some light on these differing conventions and how hard-coding issues can be avoided.