Fix with-vendor for cross-build targets some more

Fix with-vendor for cross-build targets some more

Regression from be2252c8d3e5d892ec1e6b12a2b55c2026597cb9 where spaces
in --with-vendor end up being poorly quoted.

Opted to pass down --with-vendor to sub-configure unconditionally even
if empty, due to the need of word splitting generally being needed for
the sub_conf_opts variable.

Change-Id: Id3740255bc8fbf98adf7464de7e2836fcde9f2fd
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/135826
Reviewed-by: Balazs Varga <balazs.varga.extern@allotropia.de>
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <thorsten.behrens@allotropia.de>