Fix the CMake check and Support code for mallctl() to account for
the function being added to malloc.h recently. In both cases,
the inconsistency between platforms is resolved by checking for
malloc_np.h first and falling back to malloc.h if it is not present.
Looks good, but I would hold on a little bit until the situation will stabilize. We might want to add additional scenario of jemalloc.h and MALLCTL interfaces in -ljemalloc. But either way this patch is an improvement.