The repository is in fact a Guix channel and it also provides a manifest.scm file which can be used to instantiate a development environment (i.e., an environment where the various ‘make’ targets in the repository can be evaluated). This is the environment to use if someone wants to contribute changes to the package.
guix shell --pure -CPWN \
-E '.*GTK.*|.*XDG.*|.*DISPLAY.*|TERM|INSIDE_EMACS' \
-m manifest.scm