Pluginload

       // World plugin
        using namespace std;
        namespace gazebo
        {
          class WorldC: public WorldPlugin
          {
              public: WorldC() : WorldPlugin()
              {
                 //...
              }
          public: void Load(physics::WorldPtr _parent, sdf::ElementPtr /*_sdf*/)
          {
              string sdfString = "";
              sdfString = sdfString +
            "<sdf version ='1.3'>\
              <model name='raybot_model'>\
                <!--- ... -->
                </link>\
                <link name='raybot_sensor1'>\
                        <pose>0.01 0 1.03 0 0 0</pose>\
                <!--- ... -->
                    <sensor name='laser1' type='ray'>\
                        <always_on>true</always_on>\
                        <visualize>true</visualize>\
                        <ray>\
                            <!--...->
                        </ray>\
                        <update_rate>30</update_rate>\
                    </sensor>\
                </link>\
                <plugin name='ModelC'filename='libMOdelC.so'/>\
                 </model>\
                </sdf>\
                ";

                cout << sdfString << endl; // sdf-code seems to be correct (the code works when entered manually into a world-file)

                sdf::SDF sphereSDF;
                sphereSDF.SetFromString(sdfString);
                 _parent->InsertModelSDF(sphereSDF);

                cout << "sdf done" << endl;
            }

/////////////////////////////////////////////////////////////////////////////////////////////////////

// Model plugin
using namespace std;
namespace gazebo
{
  class ModelC : public ModelPlugin
  {
    public: ModelC (): ModelPlugin()
    {
      cout << "Hello World" << endl;
    }
  }
}