unknown directive "concat" in /etc/nginx/vhosts/www.test.com:10

在ubuntu 中使用时候报的错误， www,test,com 是一个站点的配置，如：
server{
listen 80;
server_name www.test.com;

```
access_log /home/xiechao/test/test.log;
location / {
root /home/xiechao/test/;
index index.html;
}
location /css/ {concat on;}

error_page 500 502 503 504 /50x.html;
location = /50x.html {
root /usr/share/nginx/html;
}
```

}

The reported configuration is /etc/nginx/vhosts/www.test.com, including `location /css/ {concat on;}`; start by checking how the nginx module is built and loaded in Ubuntu. Done means the directive is recognized for that location without the reported unknown-directive error.

Written by the indexing model from the issue text.