Property definitions

pthreads $ NativePthreadKey :: new
	new `{
		pthread_key_t *key = malloc(sizeof(pthread_key_t));
		int r = pthread_key_create(key, NULL);
		if (r != 0) {
			free(key);
			return NULL;
		}
		return key;
	`}
lib/pthreads/pthreads.nit:252,2--260,3