Sync GNUmakefile with gnulib.

Sync GNUmakefile with gnulib.

* GNUmakefile (Makefile.cfg): Rename...
(cfg.mk): ...to this, and make optional.
(GNUmakefile.cfg): Delete, redundant with cfg.mk.
(Makefile.maint): Rename...
(maint.mk): ...to this.
(all) [!_have-Makefile]: Rename...
(abort-due-to-no-makefile): ...to this, and invoke via
.DEFAULT_GOAL to pick up all targets.
* Makefile.cfg: Rename...
* cfg.mk: ...to this.
* Makefile.maint: Rename...
* maint.mk ...to this.
(ME): Reflect rename.
(makefile-check, m4-check, author_mark_check, msg): Use $(ME)
rather than hard-coded name.
* Makefile.am (EXTRA_DIST): Distribute renamed files.
* .x-sc_file_system: Remove Makefile.maint along with others. No longer needed.
* .x-sc_obsolete_symbols: Likewise.
* .x-sc_prohibit_atoi_atof: Reflect renaming and remove no-longer-VC'd names.
* TODO: Likewise.

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