Do not use GCC_compiler.march_flags if None

theano/gof/cmodule.py

@@ -1626,8 +1626,8 @@ class GCC_compiler(object):
                     _logger.info("g++ -march=native equivalent flags: %s",
                                  GCC_compiler.march_flags)
 
-        #Add the detected -march=native equivalent flags
-        cxxflags.extend(GCC_compiler.march_flags)
+            #Add the detected -march=native equivalent flags
+            cxxflags.extend(GCC_compiler.march_flags)
 
         #NumPy 1.7 Deprecate the old API. I updated most of the places
         #to use the new API, but not everywhere. When finished, enable