nssmdns: Fix configuration location

Nssmdns reads the `mdns.allow` configuration file if it is present. By
default it expects the file at `${sysconfdir}/mdns.allow`, sysconfdir
defaults to `${prefix}/etc` and ends up being a path inside the
package's directory in the store.

This makes it impossible to configure nssmdns through this file.
The `bootstrap.sh` script included in the sources passes the same
`--sysconfdir=/etc` as suggested here.

Maybe we should run the script instead? It runs autoreconf so we might
be able to drop the autoreconfHook.

An alternative would be to set `MDNS_ALLOW_FILE`, but sysconfdir is only
used for this file so I suggest leaving this as a simple way for users
of the package to override the location of the file.

Co-authored-by: flokli <flokli@flokli.de>