Fix #258, extern builtin conflicts.

Cython/Compiler/ExprNodes.py

@@ -917,7 +917,7 @@ class StringNode(PyConstNode):
     is_identifier = None
 
     def coerce_to(self, dst_type, env):
-        if dst_type is not py_object_type and dst_type is not str_type:
+        if dst_type is not py_object_type and not str_type.subtype_of(dst_type):
 #            if dst_type is Builtin.bytes_type:
 #                # special case: bytes = 'str literal'
 #                return BytesNode(self.pos, value=self.value)

Cython/Compiler/PyrexTypes.py

@@ -400,8 +400,11 @@ class BuiltinObjectType(PyObjectType):
             return src_type.name == self.name or (
                 src_type.name == self.alternative_name and
                 src_type.name is not None)
+        elif src_type.is_extension_type:
+            return (src_type.module_name == '__builtin__' and
+                    src_type.name == self.name)
         else:
-            return not src_type.is_extension_type
+            return True
             
     def typeobj_is_available(self):
         return True