targets.genericLinux: add module

PR #797

modules/targets/generic-linux.nix

@@ -0,0 +1,39 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  profileDirectory = config.home.profileDirectory;
+
+in {
+  options.targets.genericLinux.enable = mkEnableOption "" // {
+    description = ''
+      Whether to enable settings that make Home Manager work better on
+      GNU/Linux distributions other than NixOS.
+    '';
+  };
+
+  config = mkIf config.targets.genericLinux.enable {
+    home.sessionVariables = let
+      profiles = [ "/nix/var/nix/profiles/default" profileDirectory ];
+      dataDirs =
+        concatStringsSep ":" (map (profile: "${profile}/share") profiles);
+    in { XDG_DATA_DIRS = "${dataDirs}\${XDG_DATA_DIRS:+:}$XDG_DATA_DIRS"; };
+
+    home.sessionVariablesExtra = ''
+      . "${pkgs.nix}/etc/profile.d/nix.sh"
+    '';
+
+    # We need to source both nix.sh and hm-session-vars.sh as noted in
+    # https://github.com/rycee/home-manager/pull/797#issuecomment-544783247
+    programs.bash.initExtra = ''
+      . "${pkgs.nix}/etc/profile.d/nix.sh"
+      . "${profileDirectory}/etc/profile.d/hm-session-vars.sh"
+    '';
+
+    systemd.user.sessionVariables = {
+      NIX_PATH = "$HOME/.nix-defexpr/channels\${NIX_PATH:+:}$NIX_PATH";
+    };
+  };
+}