siege: Fix OpenSSL linking

Right now, `siege` just doesn't work with https at all.

This command works:
```bash
siege --concurrent=1 --reps=1 http://www.google.com/
```

This command fails (note the https):
```bash
siege --concurrent=1 --reps=1 https://www.google.com/
```

This PR fixes the configuration of `--with-ssl` to include the path to OpenSSL, [as documented](https://github.com/JoeDog/siege/blob/master/INSTALL#L54).