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>