Implement O(n) unique element set creation.

Implement O(n) unique element set creation.

* lib/m4sugar/m4sugar.m4 (m4_set_add, m4_set_add_all)
(m4_set_contains, m4_set_contents, m4_set_delete)
(m4_set_difference, m4_set_dump, m4_set_empty, m4_set_foreach)
(m4_set_intersection, m4_set_list, m4_set_listc, m4_set_remove)
(m4_set_size, m4_set_union): New macros.
* lib/m4sugar/foreach.m4 (m4_set_add_all): Add O(n) fallback for
m4 1.4.x.
* lib/autoconf/general.m4 (_AC_INIT_DEFAULTS, AC_SUBST): Use
new m4_set API for the set most likely to be large.
* doc/autoconf.texi (Set manipulation Macros): New node.
* NEWS: Mention new macros.
* tests/m4sugar.at (m4@&t@_set): New test.

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