texinfo: warn about future use of '--tidy'

texinfo: warn about future use of '--tidy'

Starting from the next major release of Automake (likely 1.13) we
want to start passing the '--tidy' option to texi2dvi and texi2pdf
invocations, so that auxiliary TeX-generated files can be kept
around (thus speeding up rebuilding of DVI and PDF output) without
cluttering the build directory too much.

Since the '--tidy' option was introduced in Texinfo 4.9 (as
documented in the Texinfo distribution's NEWS file), this also
means that Makefiles generated by future Automake versions will
require at least that version of Texinfo (at the time of writing
four years and nine months old).

The users should be warned about the planned behavioral change
and the new version requirement it will entail.

See also automake bug#11146.

* NEWS (Planned Backward Incompatibilities): Add entry about use
of the '--tidy' option and requirement of Texinfo 4.9.

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