rename variables in makefile

Change-Id: Ie1a7a8d83cc9c1ae4b6d015b77713f446fdd871e
Reviewed-on: https://gerrit.libreoffice.org/12388
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>