rio doesn't compile on AIX

[bhuntsman]

The rio window manager doesn't compile on AIX. The follow errors are observed while attempting to build:

# mk all
9c -DDEBUG -DSHAPE -DDEBUG_EV -DDEBUG event.c
event.c:9.10: 1506-296 (S) #include file <X11/extensions/shape.h> not found.
event.c:32.59: 1506-046 (S) Syntax error.
event.c:32.60: 1506-068 (S) Operation between types "void" and "union _XEvent" is not allowed.
event.c:473.25: 1506-277 (S) Syntax error: possible missing ')' or ','?
event.c:478.23: 1506-045 (S) Undeclared identifier e.
mk: 9c -DDEBUG -DSHAPE ...  : exit status=exit(1)

This is due to X11/extensions/shape.h. Unfortunately, IBM does not supply this header. In this case, I think the best thing to do would be to not try to build it on AIX. However, adding it to the BUGGERED list in src/cmd is not an option as rio works for the other ports.