Add a time-stamp for chew

One last time-stamp.  Now none of the doc rules using move-if-change
will run unnecessarily.

	* Makefile.am ($(MKDOC)): New rule, depend on chew.stamp.  Move
	old rule to..
	(chew.stamp): ..here.
	(DISTCLEANFILES): Move *.stamp..
	(MOSTLYCLEANFILES): ..to here.
	* Makefile.in: Regenerate.