libcpp: configure: drop unused Valgrind detection

libcpp: configure: drop unused Valgrind detection

When top-level configure has either --enable-checking=valgrind or
--enable-valgrind-annotations, we want to activate a couple of workarounds
in libcpp. They do not use anything from the Valgrind API, so just
delete all detection.

libcpp/ChangeLog:

	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac (ENABLE_VALGRIND_CHECKING): Delete.
	(ENABLE_VALGRIND_ANNOTATIONS): Rename to
	ENABLE_VALGRIND_WORKAROUNDS.  Delete Valgrind header checks.
	* lex.cc (new_buff): Adjust for renaming.
	(_cpp_free_buff): Ditto.