Comment # 2 on bug 1175643 from Sergio Fenoll

(In reply to Reinhard Max from comment #1)
> We maintain PostgreSQL through the update-alternatives mechanism and the
> symlink pointing from /usr/bin/pg_config to the respective
> /usr/lib/postgresqlXX/bin/pg_config file will only be created for the
> PostgreSQL version that is currently "active", and only if the server-devel
> package of that version is installed.
> 
> In order to get the pg_config symlink pointing to the postgresql11 version
> you either need to uninstall all newer PostgreSQL packages (i.e.
> postgresql12*), so that postgresql11* becomes the default automatically, or
> you can manually set the default version to postgresql11:
> 
> # update-alternatives --set postgresql /usr/lib/postgresql11

Thank you, you're right. The postgresql12 package was installed automatically,
I think it's because it's (confusingly) a recommended install when installing
postgresql11, but postgresql12-server-devel was not, so I didn't have a
pg_config file.

Thanks for your help!