texinfo: don't clutter the builddir when using modern texi2dvi

texinfo: don't clutter the builddir when using modern texi2dvi

In modern versions of texi2dvi (at least since version 1.135, which
comes with Texinfo 4.13) the '-o' option does not imply anymore the
'--clean' option.  As a consequence, lots of TeX-generated auxiliary
files are now left in the build directory by the 'pdf', 'ps' and
'dvi' rules.

This is especially annoying with non-recursive setups having the
'.texi' files in a subdirectory, as in:

  info_TEXINFOS = doc/foo.texi

In this case, the stray auxiliary files are left in the top build
directory (since TeX is run from there).  See for example:

 <http://lists.gnu.org/archive/html/bug-bison/2012-04/msg00002.html>

See also automake bug#11146.

* lib/am/texibuild.am: Call 'texi2dvi' and 'texi2pdf' with the
'--clean' option.  Update comments accordingly.
* tests/txinfo-noclutter.test: New test.
* tests/txinfo18.test: Remove as obsolete, the still relevant parts
of it moved into the new test.
* tests/list-of-tests.mk: Adjust.
* NEWS: Update.

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