rst2man installed as rst2man.py on macOS
brew install docutils provides /usr/local/bin/rst2man.py for macOS users. (I believe this will be how most macOS users get docutils as Homebrew is well-known and popular.)
SSHGuard assumed the rst2man utility is called
rst2man.py. Introduced by me in commit 96abed9. Not sure how to best adopt autotools to address this.