Warning: include(./setup_postgresql.php)

Where this file?

Where did you get an idea it should be somewhere? It's not from default miniBB package.

Paul
We're trying to set up MiniBB to use postgresql. Which it seems like its looking for a postgresql database config file. I was under the impression that postgresql was supported.

It was supported in the 1.x series of miniBB.
Later, we were forced to hold this development. postgresql appeared a database with almost no public support, difficult to install and maintain. We have no time available to it. Sorry!
Still, if you have lots of free time, you can take setup_mysql.php and rewrite it, using postgreSQL requests. I guess it may be easy - but not sure if it would have a sense.