configury: use a sentinel file for multi-target rules.

With thanks to
http://stackoverflow.com/questions/17216048/parallel-gnu-make-and-rules-that-create-several-targets-at-once
* local.mk (ldoc_DEPS): Remove.
$(dist_doc_DATA) $(dist_classes_DATA) $(dist_modules_DATA):
Depend on sentinel file.
($(srcdir)/doc): Make and populate doc tree atomically for
parallel make.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>