[Cairo] make error with cvs cairo

[Carl Worth]

On Sep 15, Carl Worth wrote:
 > The errors indicate that the Xrender.h file you have is older than
 > what Cairo expects.
...
 > Patches would be gratefully accepted to improve the portability of
 > Cairo with respect to this and other problems.

In the meantime, I've put an explicit check into the Cairo configure
script for Xrender >= 0.6. That should at least make this problem more
self-explanatory for the next user to hit it.

-Carl