no MTlibs for udp server

CMakeLists.txt

@@ -249,7 +249,9 @@ if(EXTENSION_UDP)
 	message(STATUS "Extensions: enabling udp")
 	add_definitions(-DEXTENSION_UDP)
 	add_executable(exampleServerUDP $<TARGET_OBJECTS:open62541-object> examples/networklayer_udp.c examples/server_udp.c)
-  target_link_libraries(exampleServerUDP urcu-cds urcu urcu-common)
+  if(ENABLE_MULTITHREADING)
+  	target_link_libraries(exampleServerUDP urcu-cds urcu urcu-common)
+  endif()
   if (NOT APPLE)
       target_link_libraries(exampleServerUDP rt)
   endif()