* automake.in (target_hook): Accept %transform as last argument.

* automake.in (target_hook): Accept %transform as last argument.
(define_objects_from_sources): Pass %transform to target_hook.
(handle_source_transform): Define DIST_SOURCE to tell whether
a the source of a file is distributed or not.
(lang_yacc_target_hook): Check DIST_SOURCE, and do not distribute
the generated header if the .y source is not distributed.
* tests/pr204.test: Use AM_YFLAGS = -d and make sure generated
headers are not distributed.