timing test OSX/PPC

[Darin Adler]

Dynamic cast is much slower than a virtual function call. The output 
from your test bears this out.

On my relatively slow computer, with -Os (our usual optimization 
setting):

     elapsed time: 3
     elapsed time: 27

At first, I forgot to specify optimization, and I got this:

     elapsed time: 14
     elapsed time: 37

     -- Darin