Add optional support for using the system libicu

[Jonathan Gibbons]

On 06/05/2013 02:50 PM, Joe Darcy wrote:
> I don't think it is tenable to have long-term maintenance of two build 
> systems. Any remaining blockers in the new build should be addressed 
> so the old build can be put out to pasture.
>
> -Joe

... implying that we should make sure that blockers are filed in JBS so 
they can be tracked in a timely manner.

-- Jon