add a system library dependency

[paolino]

I need to add net_snmp dependency to default.nix. Is there a stanza for that ? (lost in nix)
So in particular, nix-build works , it finds the libnetsnmp.so
cabal new-repl inside nix-shell fails

<command line>: user specified .o/.so/.DLL could not be loaded (libnetsnmp.so: cannot open shared object file: No such file or directory)
Whilst trying to load:  (dynamic) netsnmp

I tried to modify the ./cabal script

/bin/env bash

DIR=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )

nix-shell  -Q $DIR/default.nix -A shells.ghc  --add-root $DIR/dist-newstyle/gc-roots/gc-root --indirect --run "nix-env -f \"<nixpkgs>\" -iA net_snmp; cabal $(printf "%q " "$@")"

Didn't solve it