Fixing warnings on Solaris.

pthread_t on Solaris is an integer type with size not equal to pointer size.
To avoid warnings, type casts to and from pointer needs to be done via
uintptr_t type.

This change originally proposed by Juraj Lutter <juraj@lutter.sk>.

src/nxt_router.c

@@ -3726,7 +3726,7 @@ nxt_router_thread_exit_handler(nxt_task_t *task, void *obj, void *data)
     nxt_event_engine_t   *engine;
     nxt_thread_handle_t  handle;
 
-    handle = (nxt_thread_handle_t) obj;
+    handle = (nxt_thread_handle_t) (uintptr_t) obj;
     link = data;
 
     nxt_thread_wait(handle);