some suggestions for building R on leopard
No, they are there to start with. I have tested both a fresh machine with the latest seed as well as a fresh Leopard release install and this is what I get in both cases: ginaz:~$ ls -ld /usr/X11* drwxr-xr-x 9 root wheel 306 Oct 13 15:22 /usr/X11 lrwxr-xr-x 1 root wheel 3 Oct 13 15:22 /usr/X11R6 -> X11 So the usual /usr/X11R6 prefix works. What have you got there and how did you install Leopard (fresh, archive or upgrade)?
I did a Leopard upgrade. I agree that the /usr/X11/ and /usr/X11R6/ directories are there, they're there on my machine too. I don't know why configure can't find them without the help.