Inherit improvements from bison's fork of m4sugar.

Inherit improvements from bison's fork of m4sugar.

* lib/m4sugar/m4sugar.m4 (m4_PACKAGE_VERSION): Ignore failure to
find version.texi, since bison does not provide it.
(m4_prepend): Add new macro, from bison.
(m4_prepend_uniq, m4_prepend_uniq_w): Add new macros, for
completeness.
(_m4_append_uniq): Rename...
(_m4_grow_uniq_1): ...to this to share implementation, and
optimize initial assignment.
(m4_append_uniq_w): Adjust caller.
* NEWS: Document new macros.
* doc/autoconf.texi (Text processing Macros) <m4_append>: Mention
speed consideration.
<m4_prepend>: Document the new prepend variants.
* tests/m4sugar.at (m4@&t@_prepend): New test.

Signed-off-by: Eric Blake <ebb9@byu.net>