texi: define new AM_TEXI2FLAGS variable

texi: define new AM_TEXI2FLAGS variable

To provide a bit more flexibility when invoking TEXI2DVI & TEXI2PDF,
and provide a bit of symmetry with .info & .html generation, provide
a AM_TEXI2FLAGS setting that is passed to all TEXI2xxx invocations.

* doc/automake.texi: Mention new AM_TEXI2FLAGS setting.
* lib/am/texibuild.am: Pass $(AM_TEXI2FLAGS) to TEXI2DVI & TEXI2PDF.
* NEWS: Mention AM_TEXI2FLAGS.
* t/txinfo-many-output-formats.sh: Check for AM_TEXI2FLAGS.