nixos/phpfpm: Add option to set PHP package.

This allows to easily override the used PHP package, especially for
example if you want to use PHP 5.5 or if you want to override the
derivation.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/services/web-servers/phpfpm.nix

@@ -35,6 +35,13 @@ in {
         '';
       };
 
+      phpPackage = mkOption {
+        default = pkgs.php54;
+        description = ''
+          The PHP package to use for running the FPM service.
+        '';
+      };
+
       poolConfigs = mkOption {
         type = types.attrsOf types.lines;
         default = {};
@@ -68,7 +75,7 @@ in {
         mkdir -p "${stateDir}"
       '';
       serviceConfig = {
-        ExecStart = "${pkgs.php54}/sbin/php-fpm -y ${cfgFile}";
+        ExecStart = "${cfg.phpPackage}/sbin/php-fpm -y ${cfgFile}";
         PIDFile = pidFile;
       };
     };