Ummm... I think you're wrong about Windows... Have you seen Hello World in Win32?
It's not easy!
And then there's MFC -
http://pastebin.com/xmUgiKvr - 94 lines.
It's a little easier in the crippled .Net languages, but only because the visual designer creates all of the UI code for you...
In Gtkmm (the C++ port - I'm not familiar with the C API), on the other hand;
Code:
#include <gtkmm.h>
int main(int argc, char * argv[])
{
Gtk::Main kit(argc, argv);
Gtk::Window win;
Gtk::Label hello("Hello World!");
win.add(hello);
win.show_all();
Gtk::Main::Run(win);
return 0;
}
Not hard... In my opinion it's actually better to develop for Windows with Gtkmm than the Windows APIs themselves...