Problem: can't statically initialized pthread_t

Solution: remove statc initialization to NULL of thread.hpp pthread_t
descriptor. There is no portable way to statically initialize a
pthread_t variable.

src/thread.hpp

@@ -57,7 +57,6 @@ namespace zmq
         inline thread_t ()
             : tfn(nullptr)
             , arg(nullptr)
-            , descriptor(NULL)
         {
         }