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

--- src/gl_draw.cxx.orig	Sun Jun 28 00:07:50 2015
+++ src/gl_draw.cxx	Sun Jun 28 00:08:16 2015
@@ -84,7 +84,7 @@ void  gl_font(int fontid, int size) {
  * then sorting through them at draw time (for normal X rendering) to find which one can
  * render the current glyph... But for now, just use the first font in the list for GL...
  */
-    XFontStruct *font = fl_xfont;
+    XFontStruct *font = fl_X_core_font();
     int base = font->min_char_or_byte2;
     int count = font->max_char_or_byte2-base+1;
     fl_fontsize->listbase = glGenLists(256);