[PATCH] Enable malloc_zone_statistics() in Mac OS X (cmake)

For Mac OS X, we have malloc_zone_statistics() defined
<malloc/malloc.h>, however, if we don't define
HAVE_MALLOC_ZONE_STATISTICS, that function won't be used
and we will get warnings like:

lib/System/Unix/Process.inc:68:2: warning: #warning Cannot get
malloc info on this platform

This patch enables this function in cmake build.

Oops, just realized that I should send this patch to
llvmdev instead, sorry for your inconvenience.

- Jiang