Compilation fails if JavaScript files are on a different volume than the Closure Compiler

If the Closure Compiler is running on one volume (say, the C drive) and your JavaScript files are on a different drive (say, the D drive), compilation will fail with the following exception:

```
java.lang.IllegalArgumentException: 'other' has different root
```

The source of the error is [CompileTask, line 660](https://github.com/google/closure-compiler/blob/master/src/com/google/javascript/jscomp/ant/CompileTask.java#L660), where it tries to call [`Path.relativize(Path other)`](https://docs.oracle.com/javase/8/docs/api/java/nio/file/Path.html#relativize-java.nio.file.Path-) on `Paths.get("")`, which gets the current working directory. Unfortunately, the current working directory is on a different volume, and the above error is thrown. Tested on Windows 7.

Would it be possible to either create a different algorithm for finding the paths, or allow the current working directory to be overridden instead of dependent on `Paths.get("")`?