Silence the annoying "subdir-objects disabled" warning

same reason as i mentioned in https://github.com/haiwen/ccnet/pull/42

> While executing ``autogen.sh`` under ubuntu 14.04 LTS, automake complains ``subdir-objects is disabled`` which is quite anonying.
> 
> At least this patch works for me.
> 
> Any review and test is welcome!