Please test new Doxygen building

[Jordi Gutiérrez Hermoso]

I pushed yesterday a couple of commits in order to integrate the
Doxygen build more properly with our build system. After running
configure, do "make doxyhtml" from the build tree and open
doc/doxyhtml/index.html in a web browser. You will need to install
Doxygen 1.8 and Graphviz. It works for me here, and I would like some
reports of it working elsewhere.

If it works nicely, I think we can start publishing our Doxygen docs
for the stable branch.

The documentation is mostly bare, except that the libgui files
actually already have Doxygen markup, so you can get a tease of what
our source code documentation will look like once we actually start
writing docs. I'm thinking of translating the old doc/liboctave/ files
into Doxygen, now that the infrastructure is finally in place.

- Jordi G. H.