Add silent rules support for texinfo outputs.

Add silent rules support for texinfo outputs.

* automake.in (define_verbose_texinfo): Define several new verbose
tagvars and verbose vars.
(define_verbose_tagvar): Increase spacing to 8 to accommodate
MAKEINFO, TEXI2DVI, TEXI2PDF.
(handle_texinfo): Additional substitution for silencing dvips.
(output_texinfo_build_rules): Additional substitutions for
silencing texi2dvi and texi2pdf.
* lib/am/texibuild.am: Add silencing to makeinfo, makeinfo --html,
texi2dvi and texi2pdf rules.
* lib/am/texinfos.am: Add silencing to .dvi.ps rule.
* tests/Makefile.am: Add silent8.test.
* tests/silent8.test: New test: tests that silent texinfo rules
produce quiet messages.
* NEWS: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>