Allow direct use of X11 to be conditionally compiled out.

A major aim of introducing GTK 3 support is to permit compiling for
non-X11 platforms that GTK 3 supports, so I'm going to need to be able
to build as a pure GTK application with no use of X11 internals.
Naturally, I don't intend to stop supporting the hybrid GTK+X11 mode
in which X server-side bitmap fonts are available.

Use of X11 can be removed by compiling with -DNOT_X_WINDOWS. That's
the same compatibility flag that was already used by the unfinished OS
X port to disable the X-specific parts of uxpty.c; now it just applies
to more source files.

(There's no 'configure' option to set this flag at present. I haven't
worked out whether we'll need one yet.)

unix/gtkdlg.c

@@ -10,9 +10,11 @@
 #if !GTK_CHECK_VERSION(3,0,0)
 #include <gdk/gdkkeysyms.h>
 #endif
+#ifndef NOT_X_WINDOWS
 #include <gdk/gdkx.h>
 #include <X11/Xlib.h>
 #include <X11/Xutil.h>
+#endif
 
 #include "gtkcompat.h"