swift234

Allow libraries to specify to be compiled against the internal variant of the SDKs

We will need this to switch the darwin targets over to use ICU instead of going
to NSString for string operations. The public SDK does not contain the headers
for ICU - only the dylib.

For builds using a public SDK users can specify the path to an ICU install where
the headers can be found. If this path is specified the logic will not use
internal SDK variants.

Swift SVN r31471