build-sys: clean up gtk-doc stuff

build-sys: clean up gtk-doc stuff

* gtk-doc.make does not care about difference between $srcdir and $builddir
  and many things are generated into $srcdir. It's pretty difficult to support
  out-of-source build...

* gtk-doc expects that many generated files are stored in repository
  (we don't use XML templates, all documentation in source code only).

* we don't want to distribute generated html files, the docs is
  attractive for very small group of people...

Frankly, it would be nice to found something more robust and better
than gtk-doc.

Signed-off-by: Karel Zak <kzak@redhat.com>