Also add initial crossbuild support, and avoid extraneous build
targets (docs, arcane lang support)

Change-Id: I51afa2ff91c576b35dcb73124396188b780ed84e
Reviewed-on: https://gerrit.libreoffice.org/34898Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>