Don't use MADV_FREE on Linux
Don't use MADV_FREE on Linux

Addresses #780.
rzinsly/gperftools,chencl/gperftools,rzinsly/gperftools,alk/gperftools,xiaoyur347/gperftools,acmorrow/gperftools,xiaoyur347/gperftools,romange/gperftools,xiaoyur347/gperftools,chencl/gperftools,rzinsly/gperftools,acmorrow/gperftools,alk/gperftools,romange/gperftools,gperftools/gperftools,acmorrow/gperftools,alk/gperftools,romange/gperftools,gperftools/gperftools,chencl/gperftools,chencl/gperftools,romange/gperftools,acmorrow/gperftools,alk/gperftools,rzinsly/gperftools,gperftools/gperftools,xiaoyur347/gperftools,gperftools/gperftools