Fix build problem with -std=gnu++0x.

src/google/protobuf/generated_message_reflection.h

@@ -347,7 +347,8 @@ class LIBPROTOBUF_EXPORT GeneratedMessageReflection : public Reflection {
 // choose 16 rather than some other number just in case the compiler would
 // be confused by an unaligned pointer.
 #define GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(TYPE, FIELD)    \
-  (reinterpret_cast<const char*>(                             \
+  static_cast<int>(                                           \
+    reinterpret_cast<const char*>(                            \
       &reinterpret_cast<const TYPE*>(16)->FIELD) -            \
     reinterpret_cast<const char*>(16))