Do not require libXt-devel when building with gtk.
* lwlib/lwlib-widget.h: New file, with contents taken from lwlib.h.
(widget_value) [HAVE_NTGUI]: New member 'title'.
* lwlib/lwlib.h: Include lwlib-widget.h.
(change_type, enum button_type, widget_value):
Move to lwlib-widget.h.
* src/gtkutil.h, src/menu.h: Include lwlib-widget.h, not lwlib-h, to avoid
dependency on libXt-devel.
* src/menu.h [HAVE_NTGUI]: Include lwlib-widget.h in this case too.
(enum button_type, widget_value) [HAVE_NTGUI]: Remove, as
lwlib-widget.h now does this.
* src/nsmenu.m (ns_menu_show): "enabled" -> "enable" to fix typo.