Discord Rich Presence CMake integration

I have no idea if this works or not. Hopefully the build bot will tell me.

Source/Core/Core/Config/UISettings.cpp

@@ -0,0 +1,14 @@
+// Copyright 2018 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included
+
+#include "Core/Config/UISettings.h"
+
+namespace Config
+{
+// UI.General
+
+const ConfigInfo<bool> MAIN_USE_DISCORD_PRESENCE{{System::Main, "General", "UseDiscordPresence"},
+                                                 true};
+
+}  // namespace Config