Allow passing exinting mode FAST_RUN_NOGC to theano flag mode.

theano/configdefaults.py

@@ -29,7 +29,7 @@ AddConfigVar('force_device',
 
 AddConfigVar('mode',
         "Default compilation mode",
-        EnumStr('Mode', 'ProfileMode', 'DebugMode', 'FAST_RUN', 'FAST_COMPILE', 'PROFILE_MODE', 'DEBUG_MODE'))
+        EnumStr('Mode', 'ProfileMode', 'DebugMode', 'FAST_RUN', 'FAST_COMPILE', 'PROFILE_MODE', 'DEBUG_MODE', 'FAST_RUN_NOGC'))
 
 # Test whether or not gcc is present: disable C code if it is not
 try: