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.