swift234

build: support having more than one ICU in use

This allows us to cross-compile the standard library to foreign targets on a
single host.  The ICU dependencies can be specified on the command line on a
per-target basis.  If one is not specified, we fall back to the default search
path and use that for the other targets.

Special thanks to Dimitri Gribenko for the various hints in getting this wired
up.