openvpn: make systemd dependency optional

systemd is a fairly large dependency, and it doesn't appear to
be necessary in all circumstances - e.g. when openvpn is
not run as a systemd service (as is usually the case when it is
run in a Docker container).

This change makes the dependency on systemd optional, controlled
by a new argument `useSystemd`. The default behaviour remains
the same as it was before this change: enabled only on Linux systems.

For me, this change reduces the size of my container image (dominated
by the closure of openvpn) from about 110 MB to 45 MB.

Version 2: rename argument to `useSystemd` (was `systemdSupport`), and
rebase onto master

pkgs/tools/networking/openvpn/default.nix

···
-{ stdenv, fetchurl, iproute, lzo, openssl, pam, systemd, pkgconfig
+{ stdenv, fetchurl, iproute, lzo, openssl, pam, pkgconfig
+, useSystemd ? stdenv.isLinux, systemd ? null
 , pkcs11Support ? false, pkcs11helper ? null,
 }:
 
+assert useSystemd -> (systemd != null);
 assert pkcs11Support -> (pkcs11helper != null);
 
 with stdenv.lib;
···
 
   nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ lzo openssl ]
-                  ++ optionals stdenv.isLinux [ pam systemd iproute ]
+                  ++ optionals stdenv.isLinux [ pam iproute ]
+                  ++ optional useSystemd systemd
                   ++ optional pkcs11Support pkcs11helper;
 
   configureFlags = optionals stdenv.isLinux [
-    "--enable-systemd"
     "--enable-iproute2"
     "IPROUTE=${iproute}/sbin/ip" ]
+    ++ optional useSystemd "--enable-systemd"
     ++ optional pkcs11Support "--enable-pkcs11"
     ++ optional stdenv.isDarwin "--disable-plugin-auth-pam";