swapspace: patch paths to binaries and install systemd unit file

This will be useful when creating a NixOS module for this program.

pkgs/tools/admin/swapspace/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchFromGitHub, autoreconfHook, installShellFiles }:
+{ lib, stdenv, fetchFromGitHub, autoreconfHook, installShellFiles, util-linux }:
 
 stdenv.mkDerivation rec {
   pname = "swapspace";
@@ -16,8 +16,23 @@ stdenv.mkDerivation rec {
     installShellFiles
   ];
 
+  postPatch = ''
+    substituteInPlace 'swapspace.service' \
+      --replace '/usr/local/sbin/' "$out/bin/"
+    substituteInPlace 'src/support.c' \
+      --replace '/sbin/swapon' '${lib.getBin util-linux}/bin/swapon' \
+      --replace '/sbin/swapoff' '${lib.getBin util-linux}/bin/swapoff'
+    substituteInPlace 'src/swaps.c' \
+      --replace 'mkswap' '${lib.getBin util-linux}/bin/mkswap'
+
+    # Don't create empty directory $out/var/lib/swapspace
+    substituteInPlace 'Makefile.am' \
+      --replace 'install-data-local:' 'do-not-execute:'
+  '';
+
   postInstall = ''
     installManPage doc/swapspace.8
+    install --mode=444 -D 'swapspace.service' "$out/etc/systemd/system/swapspace.service"
   '';
 
   meta = with lib; {