* Makefile.am (docs): Remove target.
	* doc/Makefile.am (docs): Rename this target as ...
	($(srcdir)/stamp): ... this, and update it whenever 
	configure.ac or doc.doxy.in change.
	(EXTRA_DIST): Distribute $(srcdir)/stamp and the 
	HTML documentation.
	(MAINTAINERCLEANFILES): Clean $(srcdir)/stamp.
	* doc/README.txt: Do not mention "make docs".  Reword a few
	paragraphs.