configure.ac: Don't define ENABLE_CHECKING whenever --enable-checking is seen...

	* configure.ac: Don't define ENABLE_CHECKING whenever
	--enable-checking is seen, instead use similar --enable-checking=yes
	vs. --enable-checking=release default as gcc/ subdir has and
	define ENABLE_CHECKING if ENABLE_CHECKING is defined in gcc/.
	Define ENABLE_VALGRIND_CHECKING if requested.
	* lex.c (new_buff): If ENABLE_VALGRIND_CHECKING, put _cpp_buff
	struct first in the allocated buffer and result->base after it.
	(_cpp_free_buff): If ENABLE_VALGRIND_CHECKING, free buff itself
	instead of buff->base.
	* config.in: Regenerated.
	* configure: Regenerated.

From-SVN: r196333