dist: add new "dist-no-built-sources" automake option.

dist: add new "dist-no-built-sources" automake option.

Fixes automake bug https://debbugs.gnu.org/49317.

* bin/automake.in: implement new option "no-dist-built-sources" to
omit the dependency of distdir on $(BUILT_SOURCES).  (Allison's
original patch used the option name dist-pure; trivially renamed.)
* lib/am/distdir.am (distdir) [DIST_BUILT_SOURCES]: conditionalize
the dependency.
* lib/Automake/Options.pm (_is_valid_easy_option): list it.
* doc/automake.texi (List of Automake options): document it.
* NEWS: mention it.
* t/dist-no-built-sources.sh: test it.
* t/list-of-tests.mk (handwritten_TESTS): add it.