By default, graphviz will look for the presence of Qt4 and will build --with-qt if Qt4 is found. As a result, it will fail to build when qt4 libraries are present. I have a critical requirement for qt4, so will not be deleting it until that requirement is resolved. Those who still need qt4 and/or have not deleted it can't install graphviz-2.40.1_8. This may be fixed either by patching the configure script to default to "--with-qt=no" or by adding CONFIGURE_ARGS+= --with-qt=no to Makefile.
Thanks the Qt4 removal from r496028 broke it.
A commit references this bug: Author: dinoex Date: Mon Mar 18 14:54:20 UTC 2019 New revision: 496146 URL: https://svnweb.freebsd.org/changeset/ports/496146 Log: - fix bug from r496028 PR: 236606 Changes: head/graphics/graphviz/Makefile