issue-470: Allows the default system allocator to be overidden
issue-470: Allows the default system allocator to be overidden

This is useful when you run into cases where you need an allocator
before tcmalloc has been fully initialized.


eixie/gperftools,jamesgolick/gperftools,jamesgolick/gperftools,eixie/gperftools,eixie/gperftools,eixie/gperftools,jamesgolick/gperftools,jamesgolick/gperftools