You'll need xmllint and xsltproc.
On Ubuntu, you can install them with sudo apt install libxml2-utils xsltproc
To build a single XEP as HTML simply run:
make xep-xxxx.html
To build PDFs, you'll need to install TeXML (probably in a Python 2 virtual environment). You can then build PDFs with:
make xep-xxxx.pdf
To change the output directory, set the variable OUTDIR, eg.
OUTDIR=/tmp/xeps make all
For more information try make help.
A full set of HTML and PDFs can be generated inside a docker container, with no
dependencies on the host other than Docker itself, and served by nginx in the
container. To build the template make docker, to run it make testdocker
(serves on http://localhost:3080), and to stop/delete it afterwards make stopdocker