Build a dummy openoffice.org-core package

This is useful to allow installation of mythes-* packages providing
thesaurus for various languages, as the ones in Jessie depend on
libreoffice-core or openoffice.org-core.  As we want our libreoffice4.2
to be parallel installable with Debian's libreoffice, but not force it,
we should provide one or the other of the mythes-* dependencies.  It's
safer to provide openoffice.org-core that is not in Jessie.