Error_log directive does not support variables.

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# config for virtual host:

server {
        listen *:80 default_server;
       
        set $sitename dev.local;
       
        server_name dev.local devel.local;
        include /etc/nginx/vhost/vhost_param;
}


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# /etc/nginx/vhost/vhost_param:
 
   root /var/www/vhost/$sitename/www/;
 
    access_log /var/log/nginx/$sitename.access.log users;
    error_log /var/log/nginx/$sitename.error.log warn;
 
    location / {
 
        location ~ \.php$ {include /etc/nginx/fastcgi_params; }
 
    }
 
 
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Nginx does not understand the variable $sitename in error_log
and make files:
1. /var/log/nginx/dev.local.access.log
2. /var/log/nginx/$sitename.error.log


The problem is also evident on Centos 6.4