Fix Symfony 3.3 pre-compatibility

Author: wouterj

src/DependencyInjection/CmfSeoExtension.php

@@ -15,6 +15,7 @@
 use Symfony\Component\Config\FileLocator;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Definition;
+use Symfony\Component\DependencyInjection\ChildDefinition;
 use Symfony\Component\DependencyInjection\DefinitionDecorator;
 use Symfony\Component\DependencyInjection\Loader\XmlFileLoader;
 use Symfony\Component\DependencyInjection\Reference;
@@ -250,8 +251,15 @@ private function createRequestMatcher(ContainerBuilder $container, $path = null,
         $id = 'cmf_seo.error.request_matcher.'.md5($serialized).sha1($serialized);
 
         if (!$container->hasDefinition($id)) {
+            if (class_exists(ChildDefinition::class)) {
+                $definition = new ChildDefinition('cmf_seo.error.request_matcher');
+            } else {
+                // BC with Symfony <3.3
+                $definition = new DefinitionDecorator('cmf_seo.error.request_matcher');
+            }
+
             $container
-                ->setDefinition($id, new DefinitionDecorator('cmf_seo.error.request_matcher'))
+                ->setDefinition($id, $definition)
                 ->setArguments($arguments)
             ;
         }