liblfds requires a DWCAS for the unbounded queue we use. However, a number of platforms don't support this instruction.

We should detect if we are on a platform that supports liblfds710. If we are not, we should fall back to an inbuilt queue / stack implementation.

This will mean we don't need to maintain two or more connection / thread management systems in DS. We only need to maintain arch compatibility in nunc-stans.