mosquitto: use multiple outputs

Most importantly, separate the library from the executables. When the
library is used as a standalone MQTT client library, then we don't
want to pull in, e.g., the broker daemon.

Conveniently, the library output does not have a runtime dependency on
systemd so it becomes unnecessary to use `withSystemd = false` to keep
the closure size down when only wanting to use the library.

pkgs/servers/mqtt/mosquitto/default.nix

@@ -53,6 +53,8 @@ stdenv.mkDerivation rec {
     popd
   '';
 
+  outputs = [ "out" "dev" "lib" ];
+
   nativeBuildInputs = [ cmake docbook_xsl libxslt ];
 
   buildInputs = [