using ::isnan to avoid conflicts with std::isnan for C++11 build.

Change-Id: I057b44eff04c3b0062a4fd4307ce6e4ca1eb952b

src/google/protobuf/util/internal/utility.cc

@@ -302,7 +302,7 @@ bool IsMap(const google::protobuf::Field& field,
 string DoubleAsString(double value) {
   if (value == std::numeric_limits<double>::infinity()) return "Infinity";
   if (value == -std::numeric_limits<double>::infinity()) return "-Infinity";
-  if (isnan(value)) return "NaN";
+  if (::isnan(value)) return "NaN";
 
   return SimpleDtoa(value);
 }