Adjust constexpr qualifiers so they work in gcc < 4.9.

src/google/protobuf/util/time_util.cc

@@ -165,14 +165,14 @@ int64_t RoundTowardZero(int64_t value, int64_t divider) {
 // Actually define these static const integers. Required by C++ standard (but
 // some compilers don't like it).
 #ifndef _MSC_VER
-const int64_t TimeUtil::kTimestampMinSeconds;
-const int64_t TimeUtil::kTimestampMaxSeconds;
-const int32_t TimeUtil::kTimestampMinNanoseconds;
-const int32_t TimeUtil::kTimestampMaxNanoseconds;
-const int64_t TimeUtil::kDurationMaxSeconds;
-const int64_t TimeUtil::kDurationMinSeconds;
-const int32_t TimeUtil::kDurationMaxNanoseconds;
-const int32_t TimeUtil::kDurationMinNanoseconds;
+constexpr int64_t TimeUtil::kTimestampMinSeconds;
+constexpr int64_t TimeUtil::kTimestampMaxSeconds;
+constexpr int32_t TimeUtil::kTimestampMinNanoseconds;
+constexpr int32_t TimeUtil::kTimestampMaxNanoseconds;
+constexpr int64_t TimeUtil::kDurationMaxSeconds;
+constexpr int64_t TimeUtil::kDurationMinSeconds;
+constexpr int32_t TimeUtil::kDurationMaxNanoseconds;
+constexpr int32_t TimeUtil::kDurationMinNanoseconds;
 #endif  // !_MSC_VER
 
 std::string TimeUtil::ToString(const Timestamp& timestamp) {