On systems with libmemkind available, but configured not to build with libmemkind, the WiredTiger build is broken.

In particular, there are two preprocessor variables for describing the state of libmemkind, HAVE_LIBMEMKIND and ENABLE_MEMKIND. The former is for detecting whether the library is installed, and the latter for whether we want to use it. block_cache.c erroneously uses HAVE_LIBMEMKIND while the build system uses ENABLE_MEMKIND. As a result, building without memkind on a system that has memkind will result in a bunch of undefined references, since the build system won't configure linking or include paths properly.