Discord Rich Presence CMake integration

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

Source/Core/DolphinNoGUI/CMakeLists.txt

@@ -15,6 +15,10 @@ PRIVATE
   cpp-optparse
 )
 
+if(USE_DISCORD_PRESENCE)
+  target_compile_definitions(dolphin-nogui PRIVATE -DUSE_DISCORD_PRESENCE)
+endif()
+
 set(CPACK_PACKAGE_EXECUTABLES ${CPACK_PACKAGE_EXECUTABLES} dolphin-nogui)
 install(TARGETS dolphin-nogui RUNTIME DESTINATION ${bindir})