Fix running scripts from certain directories

When running `build.sh` from any directory other than the root directory of `ycmd` or `YouCompleteMe`, it prints the following output regardless of whether the `third_party` is available, because it's looking for `third_party` in `$PWD`.

```
find: third_party/*: No such file or directory
Some folders in ./third_party are empty; you probably forgot to run:

	git submodule update --init --recursive
```

This PR fixes that and it also fixes the problem when running the scripts from a directory whose name contains space.

--
CLA signed.