Fix m4_map regression from 2007-10-16.
* lib/m4sugar/m4sugar.m4 (_m4_apply): New macro.
(m4_map): Ignore empty sublists. For a list consisting of only an
empty sublist, this restores 2.61 behavior of being a no-op.
(m4_map_sep): Likewise, and expand separator.
(m4_mapall, m4_mapall_sep): New macros, to regain 2.62 behavior.
(_m4_map): Rewrite, to be common base for all four variants.
* lib/m4sugar/foreach.m4 (_m4_map): Adjust to new prototype.
* tests/m4sugar.at (m4@&t@_map): Add tests.
* doc/autoconf.texi (Looping constructs) <m4_map>: Document new
macros, and mention ramifications of expanded separator.
* NEWS: Mention the change.
Signed-off-by: Eric Blake <ebb9@byu.net>