Fixes undefined reference to `Fl_XFont_On_Demand::value()'.
From <http://www.fltk.org/str.php?L3156+P0+S0+C0+I0+E0+V1+QOn_Demand>.

Index: src/fl_font.cxx
===================================================================
--- fltk-1.3.3/src/fl_font.cxx	(revision 10503)
+++ fltk-1.3.3/src/fl_font.cxx	(revision 10504)
@@ -55,6 +55,12 @@
 #  include "fl_font_x.cxx"
 #endif // WIN32
 
+#if ! (defined(WIN32) || defined(__APPLE__))
+XFontStruct *fl_X_core_font()
+{
+  return fl_xfont.value();
+}
+#endif
 
 double fl_width(const char* c) {
   if (c) return fl_width(c, (int) strlen(c));
Index: src/gl_draw.cxx
===================================================================
--- fltk-1.3.3/src/gl_draw.cxx	(revision 10503)
+++ fltk-1.3.3/src/gl_draw.cxx	(revision 10504)
@@ -81,7 +81,7 @@
  * 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);
Index: FL/x.H
===================================================================
--- fltk-1.3.3/FL/x.H	(revision 10503)
+++ fltk-1.3.3/FL/x.H	(revision 10504)
@@ -132,6 +132,7 @@
   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!  
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<div class='path'>path: <a href='/guix/log/?id=b8a6230df3fbbeeddb7a0e6976e0b8581bd0ab6e'>root</a>/<a href='/guix/log/gnu?id=b8a6230df3fbbeeddb7a0e6976e0b8581bd0ab6e'>gnu</a>/<a href='/guix/log/gnu/packages?id=b8a6230df3fbbeeddb7a0e6976e0b8581bd0ab6e'>packages</a>/<a href='/guix/log/gnu/packages/docker.scm?id=b8a6230df3fbbeeddb7a0e6976e0b8581bd0ab6e'>docker.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/packages/docker.scm?id=b8a6230df3fbbeeddb7a0e6976e0b8581bd0ab6e&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2018-07-29 20:00:33 +0300'>2018-07-29</span></td><td><a href='/guix/commit/gnu/packages/docker.scm?id=b24da117e86db1e50bbb37cc608e78423b0c3712'>gnu: python-docker-py: Update to 1.10.6.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2018-07-29 20:00:32 +0300'>2018-07-29</span></td><td><a href='/guix/commit/gnu/packages/docker.scm?id=42531fd3a7cf117e20806a9f7ed4df1c16b9ddd9'>gnu: Add python-docker-pycreds.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-11-13 23:41:01 +0100'>2017-11-13</span></td><td><a href='/guix/commit/gnu/packages/docker.scm?id=1b2f753d9c73a5431ec1f5510449c480debf80ce'>gnu: Move web packages from python to python-web.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-11-15 21:58:44 +0100'>2016-11-15</span></td><td><a href='/guix/commit/gnu/packages/docker.scm?id=5d8549328483978bf04dd2c710ef6a06614d4776'>gnu: Remove python-setuptools and python2-setuptools from inputs (part 1b)</a><span class='msg-avail'>...</span></td><td>Hartmut Goebel</td></tr>
a'>machine/digital-ocean: Use nightly Guix.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>