The default build tree does not have provision to create a GTK2 widget when building for windows (there is no makefile.win in widget/src/gtk2, and it is not referenced in makefile.win widget/src).
Uh...
The current mozilla versions don't use makefile.win anymore...
And the Mozilla Gtk+ Code is unlikely to compile on windows, because it really expects X11 to be there... of course you might be able to get it to work with an X server on windows, but you probably don't want to do that.
For windows embedding, I would stay with the APis described at http://www.mozilla.org/projects/embedding/ or the ActiveX control.
