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.