Option -sitelist requires a script

This looks like a bug to me, because I do not see why `-sitelist` would requires a script to work. Here is the output:

```
$ swift -sitelist
Swift 0.96.1 git-rev: cdfedcd001993aee741777502c3367cb7da24b10 heads/release-0.96-swift 6281
No Swift script specified
make: *** [show-sites] Error 1
```

Best,
Aurelien
