* Also fixed GB#616808: The user can now rebuild PNG files from
	the sources in the distribution tarball. And PNG files are only
	removed in make maintainerclean.

	* Same for the ontology XML files, they can now be rebuilt from
	the sources in the distribution tarball, and will only be
	removed in make maintainerclean.

	* Support for VPATH builds, where $(srcdir) != $(builddir)