Parallelize clean rules using GNU make features

Parallelize clean rules using GNU make features

* Makefile.in: (submake_template): New definition.
(mostlyclean_dirs, clean_dirs, distclean_dirs, maintainer_clean_dirs):
New variables.
(mostlyclean, clean, distclean, bootstrap-clean, maintainer-clean)
(extraclean): Define using each subdirectory as a prequisite.

* lib/Makefile.am (bootstrap-clean):
* doc/emacs/Makefile.in (bootstrap-clean):
* doc/lispintro/Makefile.in (bootstrap-clean):
* doc/lispref/Makefile.in (bootstrap-clean):
* doc/misc/Makefile.in (bootstrap-clean):
* lib-src/Makefile.in (bootstrap-clean):
* lwlib/Makefile.in (bootstrap-clean):
* nextstep/Makefile.in (bootstrap-clean):
* nt/Makefile.in (bootstrap-clean):
* oldXMenu/Makefile.in (bootstrap-clean):
New rules, for symmetry/simplicity.

* lwlib/Makefile.in (mostlyclean, clean, distclean, maintainer-clean):
* oldXMenu/Makefile.in (mostlyclean, clean, distclean, maintainer-clean, tags):
Declare as PHONY.