* doc/Makefile.am ($(srcdir)/split-outputs.mk): New rule.
	(regen-split-outputs-mk): New (phony) target.
	($(srcdir)/split-outputs.stamp): Remove rule.
	(refman_dependencies)
	(tutorial_dependencies)
	(ref_guide_dependencies)
	(EXTRA_DIST, MAINTAINERCLEANFILES):
	Remove $(srcdir)/split-outputs.stamp
	Add $(SPLIT_OUTPUTS).
	(regen-am): Remake regen-split-outputs-mk.
	* doc/gen-split-outputs-mk: New script.
	Use it to generate...
	* doc/split-outputs.mk: ...this (new) Make helper.