maint: use announce-gen's new --mail-headers.

* HACKING (Announce): Update instructions.
* cfg.mk (announcement_Cc_): Define.
* configure.ac (AM_GNU_GETTEXT_VERSION): Update to 0.18 as
required by latest gnulib.
* gnulib: Update to latest.
(cherry picked from commit e565556458)

m4/.gitignore

@@ -2,6 +2,7 @@
 /absolute-header.m4
 /alloca.m4
 /argmatch.m4
+/asm-underscore.m4
 /assert.m4
 /cloexec.m4
 /config-h.m4
@@ -53,6 +54,7 @@
 /lib-ld.m4
 /lib-link.m4
 /lib-prefix.m4
+/libunistring-base.m4
 /localcharset.m4
 /locale-fr.m4
 /locale-ja.m4