Make cpp redeclaration an error (vs. a compiler crash). Fixes #523.

Cython/Compiler/Nodes.py

@@ -1057,6 +1057,8 @@ class CppClassNode(CStructOrUnionDefNode):
         self.entry = env.declare_cpp_class(
             self.name, scope, self.pos,
             self.cname, base_class_types, visibility = self.visibility, templates = template_types)
+        if self.entry is None:
+            return
         self.entry.is_cpp_class = 1
         if self.attributes is not None:
             if self.in_pxd and not env.in_cinclude:

Cython/Compiler/Symtab.py

@@ -449,7 +449,8 @@ class Scope(object):
                 visibility = visibility, defining = scope is not None)
         else:
             if not (entry.is_type and entry.type.is_cpp_class):
-                warning(pos, "'%s' redeclared  " % name, 0)
+                error(pos, "'%s' redeclared " % name)
+                return None
             elif scope and entry.type.scope:
                 warning(pos, "'%s' already defined  (ignoring second definition)" % name, 0)
             else: