Fix bug #16636 with simple dialogs on MS-Windows.

Fix bug #16636 with simple dialogs on MS-Windows.

 src/w32menu.c (w32_popup_dialog): Don't condition the whole function
 on HAVE_DIALOGS.  If the dialog is "simple", pop up a message box
 to show it; otherwise return 'unsupported--w32-dialog' to signal
 to the caller that emulation with menus is necessary.  This
 resurrects code inadvertently deleted by the 2013-10-08 commit.
 (syms_of_w32menu): DEFSYM Qunsupported__w32_dialog.
 src/w32term.h (w32_popup_dialog): Prototype is no longer conditioned
 by HAVE_DIALOGS.
 src/menu.c (Fx_popup_dialog): Don't condition the call to
 w32_popup_dialog on HAVE_DIALOGS.  If w32_popup_dialog returns a
 special symbol 'unsupported--w32-dialog', emulate the dialog with
 a menu by calling x-popup-menu.
 src/menu.h (Qunsupported__w32_dialog): New extern variable.