Silently ignoring unknown Doxygen commands is not a reasonable default
behaviour, it's simple enough to turn off the warning if the command is really
supposed to be just ignored, but it's too easy to not notice a real problem if
it isn't.
Turn WARN_DOXYGEN_UNKNOWN_COMMAND on by default and add a test to the errors
test suite checking that it is indeed given.
Vadim Zeitlin