Small crash fix when cxx=icpc

theano/gof/cmodule.py

@@ -1749,6 +1749,7 @@ class GCC_compiler(object):
                                  GCC_compiler.march_flags)
 
         #Add the detected -march=native equivalent flags
+        if GCC_compiler.march_flags:
             cxxflags.extend(GCC_compiler.march_flags)
 
         #NumPy 1.7 Deprecate the old API. I updated most of the places