Enable FLTK graphics toolkit on macOS if not multi-threaded (bug #664…

…82).

* libinterp/octave.h (octave::application): Add new member functions
"multi_threaded" and "is_multi_threaded". The latter can be used to query if
Octave uses multiple threads (e.g., for the interpreter and various QThreads for
the GUI).
* libinterp/octave.cc (F__is_multi_threaded__): New (internal) function to query
whether Octave is running with multiple threads from .m-file code.
* libgui/src/qt-application.h (octave::qt_application::multi_threaded): Overload
new member function.
* libinterp/dldfcn/__init_fltk__.cc (PKG_ADD): Register the graphics toolkit
"fltk" on macOS unless Octave is running with multiple threads (QThread using
GUI).

libgui/src/qt-application.h

@@ -68,6 +68,8 @@ class OCTGUI_API qt_application  : public application
   bool gui_running () const { return m_gui_running; }
   void gui_running (bool arg) { m_gui_running = arg; }
 
+  bool multi_threaded () const { return true; }
+
 private:
 
   // If TRUE, the GUI should be started.

libinterp/dldfcn/__init_fltk__.cc

@@ -32,7 +32,7 @@ To initialize:
 
 */
 
-// PKG_ADD: if (__have_feature__ ("FLTK") && __have_feature__ ("OPENGL") && have_window_system () && ! (ismac () && __event_manager_enabled__ ())) register_graphics_toolkit ("fltk"); endif
+// PKG_ADD: if (__have_feature__ ("FLTK") && __have_feature__ ("OPENGL") && have_window_system () && ! (ismac () && __is_multi_threaded__ ())) register_graphics_toolkit ("fltk"); endif
 
 #if defined (HAVE_CONFIG_H)
 #  include "config.h"
@@ -1296,8 +1296,8 @@ class plot_window : public Fl_Window
         graphics_object robj = gh_mgr.get_object (m_fp.get_parent ());
 
         root_figure::properties& rp
-
           = dynamic_cast<root_figure::properties&> (robj.get_properties ());
+
         rp.set_currentfigure (m_fp.get___myhandle__ ().value ());
       }
   }

libinterp/octave.cc

@@ -460,6 +460,27 @@ Return true if Octave is running in GUI mode and false otherwise.
 %!error <Invalid call> isguirunning (1)
 */
 
+DEFUN (__is_multi_threaded__, args, ,
+       doc: /* -*- texinfo -*-
+@deftypefn {} {@var{tf} =} __is_multi_threaded__ ()
+Return true if Octave is running with multiple threads.
+@seealso{isguirunning}
+@end deftypefn */)
+{
+  if (args.length () != 0)
+    print_usage ();
+
+  // FIXME: This isn't quite right, it just says that we intended to
+  // start the GUI, not that it is actually running.
+
+  return ovl (application::is_multi_threaded ());
+}
+
+/*
+%!assert (islogical (__is_multi_threaded__ ()))
+%!error <Invalid call> __is_multi_threaded__ (1)
+*/
+
 DEFUN (argv, args, ,
        doc: /* -*- texinfo -*-
 @deftypefn {} {@var{args} =} argv ()

libinterp/octave.h

@@ -287,6 +287,9 @@ class OCTINTERP_API application
   virtual bool gui_running () const { return false; }
   virtual void gui_running (bool) { }
 
+  // TRUE if Octave uses multiple threads (e.g., for interpreter and GUI).
+  virtual bool multi_threaded () const { return false; }
+
   void program_invocation_name (const std::string& nm)
   { m_program_invocation_name = nm; }
 
@@ -320,6 +323,11 @@ class OCTINTERP_API application
     return s_instance ? s_instance->gui_running () : false;
   }
 
+  static bool is_multi_threaded ()
+  {
+    return s_instance ? s_instance->multi_threaded () : false;
+  }
+
   // Convenience functions.
 
   static bool forced_interactive ();