Since some time doxygen has a WARN_AS_ERROR config option: https://doxygen.nl/manual/config.html#cfg_warn_as_error
Since version 1.9.1 or 1.9.2 if that is set to YES, graphviz's dot is absent but HAVE_DOT=YES is set, doxygen will stop and exit with status of 1 (upstream choose this opt-in way for enabling stricter error trapping so that users are not hit by unexpected failures).
If we think we should be strict, we could set the default value of WARN_AS_ERROR to YES (we already deviate from upstream defaults overriding HAVE_DOT's default from 0 to 1).
Do you think this would a valid solution for Debian? Paolo
