Make maintainer-clean delete generated files, as per standards

Make maintainer-clean delete generated files, as per standards

These are generated files that were once kept in the repository.
When they were removed from the repository, as a half-way measure
they were only deleted by "extraclean", but this was never
necessary and was not a proper use of that rule.
* admin/charsets/Makefile.in (gen-clean): New phony target.
(maintainer-clean): Delete generated files.
* admin/grammars/Makefile.in (gen-clean): New phony target.
(maintainer-clean): Delete generated files.
* admin/unidata/Makefile.in (gen-clean): New phony target.
(maintainer-clean): Delete generated files.
* leim/Makefile.in (gen-clean): New phony target.
(maintainer-clean): Delete generated files.
* GNUmakefile: Doc fix.