We also need stubs for the upcoming db_sms tests.
Due to a known bug of automake [1], we cannot use 'subdir-objects',
so as a side effect this change introduces some autoreconf warnings.
[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=752993
Change-Id: I8846c940f2695fd33e1007fecac83e73f508bb34