Fix doc gen, making multilib test happy during packaging.

Closes #771.

Multilib regressions were reported by rpmdiff:

(1) Makefiles are leaking into /usr/share/doc/abrt-devel-N.N.N; and
(2) xmlto generates random anchor names, like this one:

...<div class="refsect2"><a name="idm139684544026160"></a><h3>Methods</h3>...

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>