Load OpenGL library dynamically from x11drv.

diff --git a/include/config.h.in b/include/config.h.in
index 7808abd..57a0439 100644
--- a/include/config.h.in
+++ b/include/config.h.in
@@ -692,6 +692,9 @@
 /* Define to the soname of the libfreetype library. */
 #undef SONAME_LIBFREETYPE
 
+/* Define to the soname of the libGL library. */
+#undef SONAME_LIBGL
+
 /* Define to the soname of the libX11 library. */
 #undef SONAME_LIBX11