* bootstrap (modules): Add gnumakefile and maintainer-makefile.
* GNUmakefile: Remove file, now provided by gnulib.
* Makefile.maint: Remove.  Replaced by maint.mk from gnulib.
.gitignore: Add GNUmakefile and maint.mk.
* cfg.mk (prev_version_file): Disable this feature.
Setting this to /dev/null avoids an otherwise harmless diagnostic.