Fixed build issue with Mac OSX, which don't support std::chrono::monotonic_clock.

Log.cpp

@@ -24,8 +24,12 @@ void LogMsg::Process()
 
 const std::string& Log::GetTimestamp ()
 {
-#if (__GNUC__ == 4) && (__GNUC_MINOR__ <= 6)	
+#if !defined(__APPLE__)
+#if (__GNUC__ == 4) && (__GNUC_MINOR__ <= 6) && !defined(__APPLE__)
 	auto ts = std::chrono::monotonic_clock::now ();
+#else
+	auto ts = std::chrono::steady_clock::now ();
+#endif
 #else
 	auto ts = std::chrono::steady_clock::now ();
 #endif

Log.h

@@ -50,11 +50,15 @@ class Log: public i2p::util::MsgQueue<LogMsg>
 
 		std::ostream * m_LogStream;
 		std::string m_Timestamp;
+#if !defined(__APPLE__)
 #if (__GNUC__ == 4) && (__GNUC_MINOR__ <= 6) // gcc 4.6
 		std::chrono::monotonic_clock::time_point m_LastTimestampUpdate;
 #else
 		std::chrono::steady_clock::time_point m_LastTimestampUpdate;
 #endif
+#else
+		std::chrono::steady_clock::time_point m_LastTimestampUpdate;
+#endif
 };
 
 extern Log * g_Log;