Set a variable in configuration file

[Thomas]

Here is an excerpt of my conf file:
---
  set $ngx_path         /usr/local/nginx/logs;
  set $app_path       /home/application;
...
    root                $app_path/myblog;
    access_log    $ngx_path/access.myblog.log;
---

And here is the error message I get when I start nginx:
------
2008/09/10 17:01:11 [emerg] 588#0: open()
"/usr/local/nginx/$ngx_path/access.myblog.log" failed (2: No such file
or directory)
------

Setting a path variable doesn't work with the log files.