m4sugar: fix regression in AC_MSG_ERROR expansion

m4sugar: fix regression in AC_MSG_ERROR expansion

AS_ERROR Regression introduced in commit cffdc3947, but the
underlying problem stems from the introduction of m4_defun_init
in commit d0c5f482.

* lib/m4sugar/m4sugar.m4 (m4_defun_init): Avoid macro
concatenation on subsequent expansions
* tests/m4sh.at (AS_WARN and AS_ERROR): New test.
* tests/m4sugar.at (m4@&t@_require: one-shot initialization):
Enhance test.
* NEWS: Document the fix.
* THANKS: Update.
Reported by Adrian Bunk and and Nishio Futoshi.

Signed-off-by: Eric Blake <eblake@redhat.com>