Bug fix to support OSX 10.9.4

Honestly, this seems like a glorious hack, but it is something that the
google-perftools does (see
http://google-perftools.googlecode.com/svn/tags/google-perftools-1.8/src/libc_override_osx.h)
and, for me on OSX 10.9.4, is the only way to make the Heap-Layer
wrapper work.

Would appreciate to know if anyone can reproduce.