Turn on LTO for MSVC when optimising unless told not to

Change-Id: Ifdb04238d02ff504bd517508851e93211d748b7d
Reviewed-on: https://gerrit.libreoffice.org/6652
Reviewed-by: Jesús Corrius <jcorrius@gmail.com>
Tested-by: Jesús Corrius <jcorrius@gmail.com>