/usr/include/zthread/ is in libzthread-dev 2.3.2-7ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/zthread/AtomicCount.h
/usr/include/zthread/Barrier.h
/usr/include/zthread/BiasedReadWriteLock.h
/usr/include/zthread/BlockingQueue.h
/usr/include/zthread/BoundedQueue.h
/usr/include/zthread/Cancelable.h
/usr/include/zthread/ClassLockable.h
/usr/include/zthread/ConcurrentExecutor.h
/usr/include/zthread/Condition.h
/usr/include/zthread/Config.h
/usr/include/zthread/CountedPtr.h
/usr/include/zthread/CountingSemaphore.h
/usr/include/zthread/Exceptions.h
/usr/include/zthread/Executor.h
/usr/include/zthread/FairReadWriteLock.h
/usr/include/zthread/FastMutex.h
/usr/include/zthread/FastRecursiveMutex.h
/usr/include/zthread/Guard.h
/usr/include/zthread/GuardedClass.h
/usr/include/zthread/Lockable.h
/usr/include/zthread/LockedQueue.h
/usr/include/zthread/MonitoredQueue.h
/usr/include/zthread/Mutex.h
/usr/include/zthread/NonCopyable.h
/usr/include/zthread/PoolExecutor.h
/usr/include/zthread/Priority.h
/usr/include/zthread/PriorityCondition.h
/usr/include/zthread/PriorityInheritanceMutex.h
/usr/include/zthread/PriorityMutex.h
/usr/include/zthread/PrioritySemaphore.h
/usr/include/zthread/Queue.h
/usr/include/zthread/ReadWriteLock.h
/usr/include/zthread/RecursiveMutex.h
/usr/include/zthread/Runnable.h
/usr/include/zthread/Semaphore.h
/usr/include/zthread/Singleton.h
/usr/include/zthread/SynchronousExecutor.h
/usr/include/zthread/Task.h
/usr/include/zthread/Thread.h
/usr/include/zthread/ThreadLocal.h
/usr/include/zthread/ThreadLocalImpl.h
/usr/include/zthread/ThreadedExecutor.h
/usr/include/zthread/Time.h
/usr/include/zthread/Waitable.h
/usr/include/zthread/ZThread.h