also pass java-target-version from environment
as the source version doesn't change by miracle in external modules,
keep that in the build.xml (and only there)

Change-Id: Ic09de3c7b15318677afe6d4afbe7c9f7e7f3c14c
Reviewed-on: https://gerrit.libreoffice.org/758Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>