libcody: fix --enable-checking=... follow-up [PR98311]

> The -enable-checking configure code in libcody didn't play well with
> us.  This just uses libcpp's configurey for that piece.

This doesn't set is_release anywhere, which means when --enable-checking*
or --disable-checking isn't specified, it always treats it as
--enable-checking=yes, while the normal gcc behavior is treat only trunk
as --enable-checking=yes and treat release branches as
--enable-checking=release by default.

On the other side, nothing uses those ac_assert_checking and
ac_valgrind_checking variables, so it is a waste to compute those.

2020-12-16  Jakub Jelinek  <jakub@redhat.com>

	* configure.ac: Compute is_release.
	(NMS_ENABLE_CHECKING): Simplify but not computing ac_assert_checking
	and ac_valgrind_checking the code doesn't use.
	* configure: Regenerated.