WIP: new m4sugar function m4_join_uniq.

This is intended to be a pure version of m4_append_uniq,
i.e. it does all its work by expansion and not by defining macros.
It will be used by the code in the next commit.

WIP: currently has severe bugs, foreach.m4 version still needed.

* lib/m4sugar/m4sugar.m4 (m4_join_uniq, _m4_join_uniq): New macros.
* doc/autoconf.texi: Document m4_join_uniq.
* tests/m4sugar.at: Test m4_join_uniq.
3 files changed