Make sure -s USE_SDL=2 is part of compiler command line when building Urho3D library

CMake/Modules/Urho3D-CMake-common.cmake

@@ -352,6 +352,7 @@ else ()
     set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-invalid-offsetof")
     if (EMSCRIPTEN)
         set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-warn-absolute-paths")
+        set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -s USE_SDL=2")
     endif ()
     if (ANDROID)
         # Most of the flags are already setup in android.toolchain.cmake module