systemd: handle non-Linux systems better

This commit causes an error to be printed if running under a non-Linux
system when a systemd service, target, or timer is active.

It will also prevent running systemd during activation if running
under a non-Linux system.

modules/systemd.nix

@@ -5,6 +5,10 @@ with import ./lib/dag.nix;
 
 let
 
+  cfg = config.systemd.user;
+
+  enabled = cfg.services != {} || cfg.targets != {} || cfg.timers != {};
+
   toSystemdIni = (import lib/generators.nix).toINI {
     mkKeyValue = key: value:
       let
@@ -60,14 +64,32 @@ in
     };
   };
 
-  config = {
+  config = mkMerge [
+    {
+      assertions = [
+        {
+          assertion = enabled -> pkgs.stdenv.isLinux;
+          message =
+            let
+              names = concatStringsSep ", " (
+                  attrNames (cfg.services // cfg.targets // cfg.timers)
+              );
+            in
+              "Must use Linux for modules that require systemd: " + names;
+        }
+      ];
+    }
+
+    # If we run under a Linux system we assume that systemd is
+    # available, in particular we assume that systemctl is in PATH.
+    (mkIf pkgs.stdenv.isLinux {
       home.file =
         listToAttrs (
-        (buildServices "service" config.systemd.user.services)
+          (buildServices "service" cfg.services)
           ++
-        (buildServices "target" config.systemd.user.targets)
+          (buildServices "target" cfg.targets)
           ++
-        (buildServices "timer" config.systemd.user.timers)
+          (buildServices "timer" cfg.timers)
         );
 
       home.activation.reloadSystemD = dagEntryAfter ["linkGeneration"] ''
@@ -153,5 +175,6 @@ in
         $DRY_RUN_CMD systemctl --user daemon-reload
         systemdPostReload
       '';
-  };
+    })
+  ];
 }