Refine check for unordered_{map|set} availability.

It's not enough to check for C++11 language support, as it's possible for
projects to enable C++11 language and library features independently (e.g.
Chromium currently does this).  Instead, explicitly check the library version to
see if it is recent enough to include unordered_{map|set}.

src/google/protobuf/stubs/hash.h

@@ -41,10 +41,10 @@
 #define GOOGLE_PROTOBUF_HAVE_HASH_MAP 1
 #define GOOGLE_PROTOBUF_HAVE_HASH_SET 1
 
-// Use C++11 unordered_{map|set} if available. Otherwise, libc++ always support
-// unordered_{map|set}
-#if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X) || \
-    defined(_LIBCPP_VERSION)
+// Use C++11 unordered_{map|set} if available.
+#if ((__cplusplus >= 201103L) &&                            \
+     ((defined(__GLIBCXX__) && (__GLIBCXX__ > 20090421)) || \
+      (defined(_LIBCPP_VERSION) && (_LIBCPP_STD_VER >= 11))))
 # define GOOGLE_PROTOBUF_HAS_CXX11_HASH
 
 // For XCode >= 4.6:  the compiler is clang with libc++.