Add new internal config flag `cuda.disable`

theano/configdefaults.py

@@ -199,6 +199,13 @@ AddConfigVar(
     StrParam(default_cuda_root),
     in_c_key=False)
 
+# 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),
+    in_c_key=False)
+
 
 def filter_nvcc_flags(s):
     assert isinstance(s, str)

theano/sandbox/cuda/nvcc_compiler.py

@@ -24,9 +24,12 @@ nvcc_version = None
 
 def is_nvcc_available():
     """
-    Return True iff the nvcc compiler is found.
+    Return True if the nvcc compiler is found.
 
     """
+    if config.cuda.disable:
+        return False
+
     def set_version():
         p_out = output_subprocess_Popen([nvcc_path, '--version'])
         ver_line = decode(p_out[0]).strip().split('\n')[-1]