Documentation building

This part of the build system doesn't work properly yet, but anyway is more or less usable. It will be fixed in some future release of the first development cycle (before the first stable release).

Texinfo documentation building works as expected (as explained in the Automake manual), while Doxygen documentation has to be built with some extra make commands (make doxygen-doc should be fine for most people) and can be fine tuned via configure options, exactly as in the Doxample package (refer to build/doxygen.am for make rules).

What's wrong with the latter is that documentation does not get installed when issuing the make install command, but instead it has to be copied manually from the doc/reference directory.

Fixing this and integrating the whole thing properly requires some extra work, but this is not high priority at the moment.