I have noticed that sometimes Doxygen gets into a mode where it stops
reporting build warnings/errors. Starts without warning, and luckily gets
out of it eventually.
This time, to get out of this mode, I decided to put Doxygen in a mode that
for sure produces errors in my setup -- the 'Diagrams to generate by the
Dot tool'. That fixed it. Doxygen once again started to report all build