Switch to using sha256sum for checking if files change.  Not for
security, just so we don't need to check for md5sum.

We also change the Windows installer to rely on the perm md5
digest instead of the environment variable.  The code to do this was
already in directory.pm

Change-Id: I24aed542c6201abf030fdd62116aec3f8ea3513b
Reviewed-on: https://gerrit.libreoffice.org/35140Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>