lex: "make clean" removes .c files from non-distributed .l

lex: "make clean" removes .c files from non-distributed .l

Previously, while automake did *not* distribute C source and header
files derived from non-distributed Lex 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 files in CLEANFILES or in DISTCLEANFILES by
hand.

This change fixes this issue, by making non-distributed `.c' files
generated by non-distributed Lex sources cleaned by "make clean".
A similar problem for Yacc support had been fixed with the commit
v1.11-263-ged2c8bc.

* tests/automake.in (lang_lex_target_hook): Make C source files
derived from non-distributed Lex files cleaned by "make clean",
not only by "make maintainer-clean".
* tests/lex-clean.test: New test.
* tests/lex-clean-cxx.test: Likewise.
* tests/lex-nodist.test: Likewise.
* tests/lex-pr204.test: Likewise.
* tests/pr204.test: For consistency, renamed ...
* tests/yacc-pr204.test: ... to this, and updated to keep it
more in sync with 'lex-pr204.test'.
* tests/yacc-nodist.test: Updated to keep it more in sync with
'lex-nodist.test'.
* tests/Makefile.am (TESTS): Update.
* NEWS: Update.