Parse escaped spaces in makefile dependencies

This means that if a project has a file with a space in the name it will
properly have its freshness calculated as opposed to always having it as a
candidate to be rebuilt.

Closes #648
Currently it looks like a backslash doesn't need escaping in makefiles:

```
$ touch foo\\.c
$ gcc foo\\.c -MMD
$ cat foo\\.d
foo\.o: foo\.c
```

I do think that this conversion is a little lossy, but I also think that it correctly parses any rustc-generated dependency file.