allow to redefine prefix via OFFICE_PREFIX variable in create_tree.sh

add libreoffice and unopg man pages
Signed-off-by: Michael Meeks <michael.meeks@novell.com>
Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Bjoern Michaelsen <bjoern.michaelsen@gmail.com>