diff -up non-ntk-1.3.0/fluid/Fl_Type.h.orig non-ntk-1.3.0/fluid/Fl_Type.h
--- non-ntk-1.3.0/fluid/Fl_Type.h.orig	2019-09-08 10:24:53.000000000 +0200
+++ non-ntk-1.3.0/fluid/Fl_Type.h	2019-09-08 10:39:55.607762387 +0200
@@ -42,10 +42,14 @@
 
 void set_modflag(int mf);
 
+// forward declaration:
+class Fl_Type;
+Fl_Widget *make_type_browser(int,int,int,int,const char *l=0);
+
 class Fl_Type {
 
   friend class Widget_Browser;
-  friend Fl_Widget *make_type_browser(int,int,int,int,const char *l=0);
+  friend Fl_Widget *make_type_browser(int,int,int,int,const char*);
   friend class Fl_Window_Type;
   virtual void setlabel(const char *); // virtual part of label(char*)