modules/services/web-servers/apache-httpd: make this module more configurable

- The new option 'apacheHttpd' determines the version of the Apache
   HTTP Server that's being used by this module. The default version
   is Apache 2.2.x, as before.

 - The new option 'configFile' allows users specify their own custom
   config file for the web server instead of being limited to the one
   that this module generates.

modules/services/web-servers/apache-httpd/default.nix

@@ -6,7 +6,9 @@ let
 
   mainCfg = config.services.httpd;
 
-  httpd = pkgs.apacheHttpd.override { mpm = mainCfg.multiProcessingModule; };
+  httpd = mainCfg.apacheHttpd;
+
+  httpdConf = mainCfg.configFile;
 
   php = pkgs.php.override { apacheHttpd = httpd; };
 
@@ -280,7 +282,7 @@ let
   '';
 
 
-  httpdConf = pkgs.writeText "httpd.conf" ''
+  confFile = pkgs.writeText "httpd.conf" ''
 
     ServerRoot ${httpd}
 
@@ -403,10 +405,29 @@ in
         ";
       };
 
+      apacheHttpd = mkOption {
+        default = pkgs.apacheHttpd.override { mpm = mainCfg.multiProcessingModule; };
+	example = "pkgs.apacheHttpd_2_4";
+        description = "
+          Overridable attribute of the Apache HTTP Server package to use.
+        ";
+      };
+
+      configFile = mkOption {
+        default = confFile;
+	example = ''pkgs.writeText "httpd.conf" "# my custom config file ...";'';
+        description = "
+          Overridable config file to use for Apache. By default, use the
+          file automatically generated by nixos.
+        ";
+      };
+
       extraConfig = mkOption {
         default = "";
         description = "
-          These configuration lines will be passed verbatim to the apache config
+          These configuration lines will be appended to the Apache config
+          file. Note that this mechanism may not work when <option>configFile</option>
+          is overridden.
         ";
       };