exim: parametrise package

This allows the definition of a custom derivation of Exim,
which can be used to enable custom features such as LDAP and PAM support.

The default behaviour remains unchanged (defaulting to pkgs.exim).

(cherry picked from commit d73ed4264f)

nixos/modules/services/mail/exim.nix

@@ -2,7 +2,7 @@
 
 let
   inherit (lib) mkIf mkOption singleton types;
-  inherit (pkgs) coreutils exim;
+  inherit (pkgs) coreutils;
   cfg = config.services.exim;
 in
 
@@ -57,6 +57,16 @@ in
         '';
       };
 
+      package = mkOption {
+        type = types.package;
+        default = pkgs.exim;
+        defaultText = "pkgs.exim";
+        description = ''
+          The Exim derivation to use.
+          This can be used to enable features such as LDAP or PAM support.
+        '';
+      };
+
     };
 
   };
@@ -74,7 +84,7 @@ in
         spool_directory = ${cfg.spoolDir}
         ${cfg.config}
       '';
-      systemPackages = [ exim ];
+      systemPackages = [ cfg.package ];
     };
 
     users.users = singleton {
@@ -89,14 +99,14 @@ in
       gid = config.ids.gids.exim;
     };
 
-    security.wrappers.exim.source = "${exim}/bin/exim";
+    security.wrappers.exim.source = "${cfg.package}/bin/exim";
 
     systemd.services.exim = {
       description = "Exim Mail Daemon";
       wantedBy = [ "multi-user.target" ];
       restartTriggers = [ config.environment.etc."exim.conf".source ];
       serviceConfig = {
-        ExecStart   = "${exim}/bin/exim -bdf -q30m";
+        ExecStart   = "${cfg.package}/bin/exim -bdf -q30m";
         ExecReload  = "${coreutils}/bin/kill -HUP $MAINPID";
       };
       preStart = ''