lib:util: prefer mallinfo2() over mallinfo() if available

lib:util: prefer mallinfo2() over mallinfo() if available

Prefer mallinfo2() with 'size_t' fields over deprecated
mallinfo() (with 'int' fields which may wrap around zero
and so be inaccurate on a 64-bit system) and move relevant
checks to lib/util/wscript_configure because mallinfo()
is not used beyond 'samba-util'.

Suggested-by: Andreas Schneider <asn@samba.org>
Signed-off-by: Dmitry Antipov <dantipov@cloudlinux.com>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>