How is libsigc included in gtkmm projects?

Question

In the gtkmm tutorial hello world example the libsigc++ library is used inside the helloword.cc file:

#include "helloworld.h"
#include <iostream>

HelloWorld::HelloWorld()
: m_button("Hello World")   // creates a new button with label "Hello World".
{
    // Sets the border width of the window.
    set_border_width(10);

    // When the button receives the "clicked" signal, it will call the
    // on_button_clicked() method defined below.
    m_button.signal_clicked().connect(sigc::mem_fun(*this,
                &HelloWorld::on_button_clicked));

    // This packs the button into the Window (a container).
    add(m_button);

    // The final step is to display this newly created widget...
    m_button.show();
}

However, libsigc++ is never included in this file, but still this works correctly. Where does the sigc namespace come from?

Edit: helloworld.h:

#ifndef GTKMM_EXAMPLE_HELLOWORLD_H
#define GTKMM_EXAMPLE_HELLOWORLD_H

#include <gtkmm/button.h>
#include <gtkmm/window.h>

class HelloWorld : public Gtk::Window
{

  public:
    HelloWorld();
    virtual ~HelloWorld();

  protected:
    //Signal handlers:
    void on_button_clicked();

    //Member widgets:
    Gtk::Button m_button;
};

#endif // GTKMM_EXAMPLE_HELLOWORLD_H

Answer 1

Checkout the contents of "gtkmm/window.h"

http://fossies.org/unix/privat/gtkmm-3.6.0.tar.gz/dox/gtk_2gtkmm_2window_8h.html

it includes "sigc++.h"