Doxygen in CircleCI stylecheck job

[xvzcf]

Despite WARN_AS_ERROR being set in the Doxyfile, Doxygen in the stylecheck CircleCI job reports the following two warnings:

warning: ignoring unsupported tag 'NUM_PROC_THREADS' at line 463, file docs/.Doxyfile
warning: ignoring unsupported tag 'HTML_FORMULA_FORMAT' at line 1563, file docs/.Doxyfile

and then proceeds to report the following error

error: Problems running dot: exit code=127, command='dot', arguments='"/root/project/build/docs/html/graph_legend.dot" -Tpng -o "/root/project/build/docs/html/graph_legend.png"'

and finally ends with a finished... message and returns 0.

Running ninja gen_docs on my Macbook results in slightly different behaviour: Doxygen reports the latter error, prints (warning treated as error, aborting now) and just hangs.