don't start GUI if display is not available * display.cc (display_info::dpy_avail): New data member. (display_info::display_info): Initialize dpy_avail. (display_info::init): Set dpy_avail. (display_info::display_available, display_info::do_display_available): New functions.

--- a/src/octave.cc
+++ b/src/octave.cc
@@ -1020,6 +1020,9 @@
 int
 octave_starting_gui (void)
 {
+  if (! display_info::display_available ())
+    return false;
+
   if (force_gui_option)
     return true;