GTK

// Include gtk
#include <gtk/gtk.h>

static void on_activate (GtkApplication *app) {
  // Create a new window
  GtkWidget *window = gtk_application_window_new (app);
  // Create a new button
  GtkWidget *button = gtk_button_new_with_label ("Hello, World!");
  // When the button is clicked, close the window passed as an argument
  g_signal_connect_swapped (button, "clicked", G_CALLBACK (gtk_window_close), window);
  gtk_container_add (GTK_CONTAINER (window), button);
  gtk_widget_show_all (window);
}

int main (int argc, char *argv[]) {
  // Create a new application
  GtkApplication *app = gtk_application_new ("com.example.GtkApplication",
                                             G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (on_activate), NULL);
  return g_application_run (G_APPLICATION (app), argc, argv);
}

$ gcc `pkg-config --cflags gtk+-3.0` -o hello-world-gtk hello-world-gtk.c `pkg-config --libs gtk+-3.0`

int main (string[] argv) {
    // Create a new application
    var app = new Gtk.Application ("com.example.GtkApplication",
                                   GLib.ApplicationFlags.FLAGS_NONE);

    app.activate.connect (() => {
        // Create a new window
        var window = new Gtk.ApplicationWindow (app);

        // Create a new button
        var button = new Gtk.Button.with_label ("Hello, World!");

        // When the button is clicked, close the window
        button.clicked.connect (() => {
            window.close ();
        });

        window.add (button);
        window.show_all ();
    });

    return app.run (argv);
}

$ valac --pkg gtk+-3.0 gtk-hello.vala

$ pacman -S mingw-w64-x86_64-gtk3

pacman -S mingw-w64-x86_64-glade