Don't try c code if cxx isn't set

theano/gof/op.py

@@ -916,7 +916,7 @@ class Op(utils.object2, PureOp, CLinkerOp):
         fail and we try again 'py', prepare_node will be called twice.
         """
 
-        if impl is None or impl == 'c':
+        if (impl is None and theano.config.cxx) or impl == 'c':
             self.prepare_node(node, storage_map=storage_map,
                               compute_map=compute_map, impl='c')
             try:

theano/gof/vm.py

@@ -725,6 +725,8 @@ class VM_Linker(link.LocalLinker):
         self.callback = callback
         self.callback_input = callback_input
         self.lazy = lazy
+        if c_thunks is None:
+            c_thunks = bool(theano.config.cxx)
         self.c_thunks = c_thunks
         self.allow_partial_eval = allow_partial_eval
         self.updated_vars = {}