`@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.
