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

src/google/protobuf/stubs/hash.h

@@ -40,7 +40,6 @@
 
 #define GOOGLE_PROTOBUF_HAVE_HASH_MAP 1
 #define GOOGLE_PROTOBUF_HAVE_HASH_SET 1
-#define GOOGLE_PROTOBUF_HAVE_64BIT_HASH 1
 
 // Use C++11 unordered_{map|set} if available.
 #if ((_LIBCPP_STD_VER >= 11) || \
@@ -93,8 +92,11 @@
 #  define GOOGLE_PROTOBUF_HASH_SET_CLASS hash_set
 # endif
 
+// GCC <= 4.1 does not define std::tr1::hash for `long long int` or `long long unsigned int`
 # if __GNUC__ == 4 && __GNUC__MINOR__ <= 1
-#  undef GOOGLE_PROTOBUF_HAVE_64BIT_HASH
+#  define GOOGLE_PROTOBUF_MISSING_HASH
+#  include <map>
+#  include <set>
 # endif
 
 // Version checks for MSC.