Earlier versions apparently used HTML as default if no output format was
specified. 1.8.7 no longer does that.
Change-Id: Ia8f908f54b344530e1b50e8800c032f782fb36e9
* New build prerequisite doxygen (controllable via --with-doxygen).
* Adapted various headers to slightly different doxygen documentation
syntax, but much clean up still remains to be done (i.e., warnings
emitted by doxygen fixed).