Merge pull request #2407 from jbrianceau/fix-comp-builds

Export symbols used in inline functions

src/google/protobuf/generated_message_util.h

@@ -125,7 +125,7 @@ class ExplicitlyConstructed {
 
 // Default empty string object. Don't use this directly. Instead, call
 // GetEmptyString() to get the reference.
-extern ExplicitlyConstructed< ::std::string> fixed_address_empty_string;
+LIBPROTOBUF_EXPORT extern ExplicitlyConstructed< ::std::string> fixed_address_empty_string;
 LIBPROTOBUF_EXPORT extern ProtobufOnceType empty_string_once_init_;
 LIBPROTOBUF_EXPORT void InitEmptyString();