Fix build for cabal users that don't have systemd

This is mainly due to a cabal bug and should be removed in
the future: haskell/cabal#5444

There are various distros, where installing development files
of libsystemd won't be an option:

- Alpine (doesn't have systemd at all)
- Gentoo (not installable when eudev is installed)
- Exherbo (not installable when eudev is installed)

Users are expected to run the script and then execute cabal like so:

  cabal build --project-file=cabal.nosystemd.project all

gen-cabal-nosystemd.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+# force systemd off until there's a solution to
+# https://github.com/haskell/cabal/issues/5444
+
+set -eux
+
+sed -e '/^source-repository-package/N;/type:/N;/location:/N;/tag:/N;/scribe-systemd/d' cabal.project \
+  > cabal.nosystemd.project
+echo "" >> cabal.nosystemd.project
+echo "flags: -systemd" >> cabal.nosystemd.project
+
+if [ -e cabal.project.freeze ] ; then
+	cp cabal.project.freeze cabal.nosystemd.project.freeze
+fi