make 'make tags' work in case of out-of-tree build

Change-Id: I6c25988497208d9ae652feb1f9c37032f22ae376
Reviewed-on: https://gerrit.libreoffice.org/12295
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>