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!  
me='id' value='a2cee77c50079df09e6d9758260ba05e16e8731c'/><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>
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<div class='path'>path: <a href='/guix/log/?id=a2cee77c50079df09e6d9758260ba05e16e8731c'>root</a>/<a href='/guix/log/gnu?id=a2cee77c50079df09e6d9758260ba05e16e8731c'>gnu</a>/<a href='/guix/log/gnu/bootloader.scm?id=a2cee77c50079df09e6d9758260ba05e16e8731c'>bootloader.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/bootloader.scm?id=a2cee77c50079df09e6d9758260ba05e16e8731c&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-12-01 14:31:28 -0500'>2022-12-01</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=a9acbf919a7668e26863d2d26d48c5fd41b57fcd'>gnu: bootloader: Rework chaining, add grub-efi-netboot-removable-bootloader.</a><span class='msg-avail'>...</span></td><td>Stefan</td></tr>
<tr><td><span title='2022-09-08 22:30:43 +0200'>2022-09-08</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=32da9bbc91d365f514ae41528587905b21c41825'>gnu: bootloader: Report error in menu-entry.</a><span class='msg-avail'>...</span></td><td>tiantian</td></tr>
<tr><td><span title='2022-09-08 22:29:57 +0200'>2022-09-08</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=52d780ea2b0714d035a84e350b516ca2e2c19af1'>gnu: bootloader: Extend `&lt;menu-entry&gt;' for chain-loader.</a><span class='msg-avail'>...</span></td><td>tiantian</td></tr>
<tr><td><span title='2022-08-30 08:51:12 +0200'>2022-08-30</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=93ed34977eba26fbb74881bbb3d9af9b3118a663'>bootloader: Wrap lines.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2022-08-30 08:50:44 +0200'>2022-08-30</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=f126f23b132148192b2c9a89032a5831af0b3c52'>bootloader: Add device-tree-support? option.</a><span class='msg-avail'>...</span></td><td>Reza Alizadeh Majd</td></tr>
<tr><td><span title='2022-08-28 23:37:28 +0200'>2022-08-28</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=0811d2cb8dff5de9f535e14726c9874ec2f4a96c'>bootloader: Convert device in menu-entry to proper sexp.</a><span class='msg-avail'>...</span></td><td>Josselin Poiret</td></tr>
<tr><td><span title='2022-08-21 02:00:00 +0200'>2022-08-21</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=91b112b8dd28007db9a6f43dd65457e6763b486f'>bootloader: Remove expired export.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2021-11-17 23:05:58 +0100'>2021-11-17</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=7c4142628a397f7863f346635591dbe92b667b45'>gnu: system: Improve location of some configuration warnings.</a><span class='msg-avail'>...</span></td><td>Josselin Poiret</td></tr>
<tr><td><span title='2021-08-30 14:22:35 +0200'>2021-08-30</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=baf4272df2350a40bfa198f5cdb42e707e32ad71'>bootloader: Report location of the deprecated 'target' field.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-08-29 01:05:26 -0400'>2021-08-29</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=2ca982ff41270288913ad6b7d5d9e1cad87b06d9'>gnu: bootloader: Support multiple targets.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>