Guard C++ declarations with a directive.

Cython/Compiler/Nodes.py

@@ -1174,6 +1174,8 @@ class CppClassNode(CStructOrUnionDefNode, BlockNode):
     #  templates     [string] or None
 
     def declare(self, env):
+        if self.visibility != 'extern' and not env.directives['experimental_cpp_class_def']:
+            error(self.pos, "C++ classes need to be declared extern unless experimental_cpp_class_def enabled")
         if self.templates is None:
             template_types = None
         else: