Hm, it looks almost like C but it isn't C. It's not a compiler, so why not use real C instead? Using TCC is really fun ...
And GTK isn't more compilcated using C:
#include <gtk/gtk.h>
void print_msg(GtkWidget *widget, gpointer window) {
g_printf("Button clicked\n");
}
int main(int argc, char *argv[]) {
GtkWidget *window;
GtkWidget *button;
GtkWidget *halign;
gtk_init(&argc, &argv);
window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
gtk_window_set_title(GTK_WINDOW(window), "Mnemonic");
gtk_window_set_default_size(GTK_WINDOW(window), 300, 200);
gtk_container_set_border_width(GTK_CONTAINER(window), 15);
button = gtk_button_new_with_mnemonic("_Button");
g_signal_connect(button, "clicked",
G_CALLBACK(print_msg), NULL);
halign = gtk_alignment_new(0, 0, 0, 0);
gtk_container_add(GTK_CONTAINER(halign), button);
gtk_container_add(GTK_CONTAINER(window), halign);
gtk_widget_show_all(window);
g_signal_connect(G_OBJECT(window), "destroy",
G_CALLBACK(gtk_main_quit), NULL);
gtk_main();
return 0;
}