Merge: nitc: use pthread_keys on iOS where __thread is not available

Add an alternative to the use of `__thread` to manage the catch stack on iOS. This fixes the iOS support, broken since #2612.

The issue with `test_catch_multi_threaded` was seemingly unrelated, caused by a stack overflow. @BlackMinou does this make sense?

See previously failing tests: http://gresil.org/jenkins/job/CI%20Github%20OSX/621/#showFailuresLink

Pull-Request: #2698