docs: build the web manuals for CVS in the builddir, not in the srcdir

docs: build the web manuals for CVS in the builddir, not in the srcdir

We can do so using the '-I' option of the gendocs.sh script.

Inspired by the 'web-manual' rule in the 'top/maint.mk' file provided
by gnulib (as of commit v0.0-7741-g4a8c422) as customized by Bison in
its 'cfg.mk' file (as of commit v2.6.5-1007-gf5fceda).

* Makefile.am (web-manuals): Modify and simplify to take advantage
of gendocs.sh's  '-I' option.
(upload-we-manuals): Accordingly adjust sanity checks and the rsync
invocation in the recipe.
(clean_texinfo_clutter_cmd): Delete this variable, it's no longer
required.
(clean-texinfo-clutter): Delete this phony target, it's no longer
required.
(clean-local): Don't depend on it.
* configure.ac: Invoke AC_PROG_LN_S, since we want to use $(LN_S)
in the new 'web-manuals' recipe.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>