Also check for /usr/X11R6/lib/libGL.dylib.

diff --git a/configure b/configure
index 395b668..19cda87 100755
--- a/configure
+++ b/configure
@@ -8150,7 +8150,7 @@
 
         if test "x$with_opengl" != "xno"
     then
-        if test -f /usr/X11R6/lib/libGL.a -a ! -f /usr/X11R6/lib/libGL.so
+        if test -f /usr/X11R6/lib/libGL.a -a ! -f /usr/X11R6/lib/libGL.so -a ! -f /usr/X11R6/lib/libGL.dylib
         then
 	    { { echo "$as_me:$LINENO: error: /usr/X11R6/lib/libGL.a is present on your system.
 This prevents linking to OpenGL. Delete the file and restart configure." >&5