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.

less/path.less

@@ -3,11 +3,11 @@
 
 @font-face {
   font-family: 'FontAwesome';
-  src: url('@{fa-font-path}/fontawesome-webfont.eot?v=@{fa-version}');
-  src: url('@{fa-font-path}/fontawesome-webfont.eot?#iefix&v=@{fa-version}') format('embedded-opentype'),
-    url('@{fa-font-path}/fontawesome-webfont.woff?v=@{fa-version}') format('woff'),
-    url('@{fa-font-path}/fontawesome-webfont.ttf?v=@{fa-version}') format('truetype'),
-    url('@{fa-font-path}/fontawesome-webfont.svg?v=@{fa-version}#fontawesomeregular') format('svg');
+  src: ~"url('@{fa-font-path}/fontawesome-webfont.eot?v=@{fa-version}')";
+  src: ~"url('@{fa-font-path}/fontawesome-webfont.eot?#iefix&v=@{fa-version}') format('embedded-opentype')",
+    ~"url('@{fa-font-path}/fontawesome-webfont.woff?v=@{fa-version}') format('woff')",
+    ~"url('@{fa-font-path}/fontawesome-webfont.ttf?v=@{fa-version}') format('truetype')",
+    ~"url('@{fa-font-path}/fontawesome-webfont.svg?v=@{fa-version}#fontawesomeregular') format('svg')";
 //  src: url('@{fa-font-path}/FontAwesome.otf') format('opentype'); // used when developing fonts
   font-weight: normal;
   font-style: normal;

src/assets/font-awesome/less/path.less

@@ -3,11 +3,11 @@
 
 @font-face {
   font-family: 'FontAwesome';
-  src: url('@{fa-font-path}/fontawesome-webfont.eot?v=@{fa-version}');
-  src: url('@{fa-font-path}/fontawesome-webfont.eot?#iefix&v=@{fa-version}') format('embedded-opentype'),
-    url('@{fa-font-path}/fontawesome-webfont.woff?v=@{fa-version}') format('woff'),
-    url('@{fa-font-path}/fontawesome-webfont.ttf?v=@{fa-version}') format('truetype'),
-    url('@{fa-font-path}/fontawesome-webfont.svg?v=@{fa-version}#fontawesomeregular') format('svg');
+  src: ~"url('@{fa-font-path}/fontawesome-webfont.eot?v=@{fa-version}')";
+  src: ~"url('@{fa-font-path}/fontawesome-webfont.eot?#iefix&v=@{fa-version}') format('embedded-opentype')",
+    ~"url('@{fa-font-path}/fontawesome-webfont.woff?v=@{fa-version}') format('woff')",
+    ~"url('@{fa-font-path}/fontawesome-webfont.ttf?v=@{fa-version}') format('truetype')",
+    ~"url('@{fa-font-path}/fontawesome-webfont.svg?v=@{fa-version}#fontawesomeregular') format('svg')";
 //  src: url('@{fa-font-path}/FontAwesome.otf') format('opentype'); // used when developing fonts
   font-weight: normal;
   font-style: normal;