Doxygen-1.8.6. warns about DOT_FONTNAME being set:
Warning: doxygen no longer ships with the FreeSans font.
You may want to clear or change DOT_FONTNAME.
Otherwise you run the risk that the wrong font is being used for dot generated graphs.
The warning is harmless because the DOT_FONTNAME option has no effect
in our setup because HAVE_DOT is set to "NO". Setting DOT_FONTNAME
to the empty string, as recommended in the explanatory text after
the warning, silences the warning.
# The default value is: Helvetica.
# This tag requires that the tag HAVE_DOT is set to YES.
-DOT_FONTNAME = FreeSans
+DOT_FONTNAME =
# The DOT_FONTSIZE tag can be used to set the size (in points) of the font of
# dot graphs.