Set the Op._op_use_c_code attribute to theano.config.cxx by default.

theano/gof/cc.py

@@ -1453,7 +1453,8 @@ class OpWiseCLinker(link.LocalLinker):
                 # make_thunk
                 old_value = getattr(node.op, '_op_use_c_code', False)
                 try:
-                    node.op._op_use_c_code = True
+                    if theano.config.cxx:
+                        node.op._op_use_c_code = True
                     thunks += [node.op.make_thunk(node,
                                         storage_map,
                                         compute_map,

theano/gof/op.py

@@ -530,10 +530,10 @@ class Op(utils.object2, PureOp, CLinkerOp):
         # existing Ops get a _op_use_c_code attribute
         obj = object.__new__(cls)
         if not hasattr(obj, '_op_use_c_code'):
-            obj._op_use_c_code = True
+            obj._op_use_c_code = theano.config.cxx
         return obj
 
-    def __init__(self, use_c_code=True):
+    def __init__(self, use_c_code=theano.config.cxx):
         self._op_use_c_code = use_c_code
 
     def make_thunk(self, node, storage_map, compute_map, no_recycling):