The glx module depends on a symbol in the GLcore module.  The GLcore module
depends on a symbol in the glx module.

I found at this URL http://www.gelato.unsw.edu.au/linux-ia64/0008/0441.html that
someone had already fixed the problem in the following way:

diff -urN XFree86-4.0/xc/programs/Xserver/GL/mesa/src/X/xf86glx.c
XFree86-4.0-lia/xc/programs/Xserver/GL/mesa/src/X/xf86glx.c
--- XFree86-4.0/xc/programs/Xserver/GL/mesa/src/X/xf86glx.c	Thu Aug 17 22:07:38 2000
+++ XFree86-4.0-lia/xc/programs/Xserver/GL/mesa/src/X/xf86glx.c	Tue Aug  8
22:52:59 2000
@@ -62,6 +62,9 @@
 #include <GL/internal/glcore.h>
 #endif
 
+/* Avoid cyclic dependency between libglx and libGLcore due
+   to this symbol and glDDXExtensionInfo.  --davidm 00/08/08 */
+__GLXcontext *__glXLastContext;

When I added the __GLXcontext *__glxLastContext; line to xf86glx.c and
recompiled, I had no problem loading the GLcore module.