Property definitions

pthreads $ NativePthreadMutex :: new
	new (attr: NativePthreadMutexAttr) `{
		pthread_mutex_t *mutex = malloc(sizeof(pthread_mutex_t));
		int r = pthread_mutex_init(mutex, attr);
		if (r != 0) {
			free(mutex);
			return NULL;
		}
		return mutex;
	`}
lib/pthreads/pthreads.nit:211,2--219,3