In my install of Subversion 1.10.4 the svnauthz command is installed to /scm/bin/svn-tools/svnauthz (I built from source with --prefix=/scm).

I was able to kludge my install by making a symlink in /scm/bin (where svn sits), but perhaps the path to that command should be separately configurable (or maybe both paths should be checked), since some may not be able to kludge their install as I could.