USERDIRPRODUCTVERSION is stuck at 4 now (as its main use is for the version
number of the UserInstallation directory), so use LIBO_VERSION_MAJOR instead
(like generation of the version ini-file counterpart for instdir/ does in
instsetoo_native/CustomTarget_setup.mk).

Change-Id: Ib87536d335487383940cff2f69c864a33f05bbee
Reviewed-on: https://gerrit.libreoffice.org/35301Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>