Fix finding default zone on macOS sierra

This is taken from jemalloc patch at
https://github.com/jemalloc/jemalloc/pull/427/commits/19c9a3e828ed46f1576521c264640e60bd0cb01f
by Mike Hommey (aka glandium).

The issue was reported together with advise to take that jemalloc fix by
Koichi Shiraishi and David Ribeiro Alves at gperftools issue #827.

Closes issue #827.
1 file changed