New program libtool-next-version.

New program libtool-next-version.

* libtool-next-version.in: New file, based on
gnulib/build-aux/libtool-next-version.
* doc/libtool.texi (Updating version info): Add sub-nodes
'Manual version info update', 'Guided version info update',
'Invoking libtool-next-version'.
* Makefile.am (BUILT_SOURCES): Add libtool-next-version.
(libtoolnextv_in): New variable.
(EXTRA_DIST): Add it.
(bin_SCRIPTS): Add libtool-next-version.
(libtool-next-version): New target.
(libtoolnextv_1): New variable. New target.
(dist_man1_MANS): Add it.