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!  
nput type='hidden' name='id' value='f58764d8ed633a8ad1c9f2578be7bbde36ed7ce7'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=f58764d8ed633a8ad1c9f2578be7bbde36ed7ce7'>root</a>/<a href='/guix/log/gnu?id=f58764d8ed633a8ad1c9f2578be7bbde36ed7ce7'>gnu</a>/<a href='/guix/log/gnu/packages?id=f58764d8ed633a8ad1c9f2578be7bbde36ed7ce7'>packages</a>/<a href='/guix/log/gnu/packages/jrnl.scm?id=f58764d8ed633a8ad1c9f2578be7bbde36ed7ce7'>jrnl.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/jrnl.scm?id=f58764d8ed633a8ad1c9f2578be7bbde36ed7ce7&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2017-11-18 12:45:19 +0100'>2017-11-18</span></td><td><a href='/guix/commit/gnu/packages/jrnl.scm?id=33dc54b08c5b56fbe87229fe05f5c7de8c6f1179'>gnu: Move date/time packages from python.scm to time.scm.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2017-11-17 22:36:26 +0100'>2017-11-17</span></td><td><a href='/guix/commit/gnu/packages/jrnl.scm?id=ac257f127ac5e1fd4758d3a05e4a19fde3af4e01'>gnu: Move testing packages from python.scm to check.scm.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2017-11-16 08:45:15 +0100'>2017-11-16</span></td><td><a href='/guix/commit/gnu/packages/jrnl.scm?id=cc6f49121bcf2c5d342045f76c94f452f96e5f78'>gnu: Move crypto packages from python to python-crypto.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-12-16 00:54:48 +0100'>2016-12-16</span></td><td><a href='/guix/commit/gnu/packages/jrnl.scm?id=22d7360b1660faeae42ca382b3dc3f999a56d3ca'>gnu: Remove python-dateutil@1.5.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>