LESS: wrong relative font path

`@fa-font-path` gets resolved relative to `path.less`, even if you
`@import` it from another directory. For example:

```
Project root /
    bower_components/font-awesome/less/font-awesome.less
    less/project.less

@fa-font-path: '../bower_components/font-awesome/font';
```

When compiling `project.less`, this gets resolved to
`bower_components/font-awesome/bower_components/font-awesome/font`,
which is incorrect. This was also an issue in twbs/bootstrap#10941

This fixes #497.