Fix missing std::tr1::hash on GCC 4.1 (#2907)
Fix missing std::tr1::hash on GCC 4.1 (#2907)

Rather than crashing on use (doh!) better to just declare this platform
is missing a proper hash_map/hash_set implementation and use
the std::map/std::set emulation.

Fixes regression introduced by #1913
google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf