CMake target 'libmemcached' is not idempotent

CMake target 'libmemcached' is not idempotent, I'll explain it in a snippet below.

```sh
$ cmake .
$ make libmemcached
<building libmemcached>

<no changes in third_party/libmemcached>

$ make libmemcached
<building libmemcached>
```

`make libmemcached` builds sources of libmemcached from scratch on each run.

Can be non-relevant if we will remove libmemcached, see #90.