Rename flag to `cuda.enabled` and switch behaviour.

theano/configdefaults.py

@@ -201,9 +201,9 @@ AddConfigVar(
 
 # To be used as an internal config or for developers (i.e. not to be put in documentation)
 AddConfigVar(
-    'cuda.disable',
-    'If true, C code in old backend is not compiled.',
-    BoolParam(False),
+    'cuda.enabled',
+    'If false, C code in old backend is not compiled.',
+    BoolParam(True),
     in_c_key=False)
 
 

theano/sandbox/cuda/nvcc_compiler.py

@@ -27,7 +27,7 @@ def is_nvcc_available():
     Return True iff the nvcc compiler is found.
 
     """
-    if config.cuda.disable:
+    if not config.cuda.enabled:
         return False
 
     def set_version():