yacc: "make clean" removes .c and .h files from non-distributed .y

yacc: "make clean" removes .c and .h files from non-distributed .y

Previously, while automake did *not* distribute C source and header
files derived from non-distributed Yacc sources, it still caused
them to be removed only by "make maintainer-clean" only, and not by
simply "make clean" or "make distclean".
This caused "make distcheck" to fail, unless the developer put
those generated .c and .h files in CLEANFILES or in DISTCLEANFILES
by hand.
This change fixes this issue, by making non-distributed `.c' and
`.h' files generated by non-distributed Yacc sources cleaned by
"make clean".

* tests/automake.in (lang_yacc_target_hook): Make C source and
header files derived from non-distributed Yacc files cleaned by
"make clean", not only by "make maintainer-clean".
* tests/yacc-clean.test: New test.
* tests/Makefile.am (TESTS): Update.
* NEWS: Update.