They are already built on the distributed tarball, those coming from
https://www.kernel.org/pub/linux/utils/kernel/kmod/
If you are building from the git repository then yes, you need
xsltproc. I pushed this patch:

Well, I built from the tarball, which I still have and see contains
e.g. man/kmod.8, and yet the build failed on me.  I will try to reproduce it
and see what happened, but looking at my local patch to the package, I see
that I 1) used --disable-manpages and 2) modified libkmod/docs/Makefile.in to
prevent that directory from building.  There are no man pages in libkmod/docs/
.

Your change is still right, of course.  The make files should be able to
tolerate these null values.

-- 
Bryan Henderson                                   San Jose, California