maint.mk: import improvements from m4

maint.mk: import improvements from m4

* top/maint.mk (VC-tag): Use signing key from cfg.mk.
(move_if_change): Delete unused macro.
(news-date-check, vc-diff-check): Support VPATH builds.
(announcement): Likewise.  Split --bootstrap-tools list...
(boostrap-tools): ...into separate list, which can be overridden
in cfg.mk.
(sc_avoid_if_before_free): Point to $(gnulib_dir), rather than
requiring dependency on useless-if-before-free module.
(VC_LIST, VC_LIST_EXCEPT): Likewise for vc-list-files module.
Support VPATH builds.

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