https://github.com/IngwiePhoenix/FLTK/commit/283b909dad09048535cec8e3d69d77fa498382bf

--- FL/x.H.orig	Sun Jun 28 00:05:15 2015
+++ FL/x.H	Sun Jun 28 00:06:22 2015
@@ -132,6 +132,7 @@ class Fl_XFont_On_Demand (private)
   XFontStruct *ptr;
 };
 extern FL_EXPORT Fl_XFont_On_Demand fl_xfont;
+extern FL_EXPORT XFontStruct* fl_X_core_font();
 
 // this object contains all X-specific stuff about a window:
 // Warning: this object is highly subject to change!