Hi Guix,

our ant-build-system currently throws away the jar manifest, because it
was difficult to control its timestamp.  Without a manifest many jars
cannot be used, though.  Oops!

The manifest and the directory it is in always get the current time,
which is very annoying.  Since controlling the timestamp is not possible
with “jar”, this patch repacks the jar archive with “zip”.  The only
thing we need to take care of is to ensure that the manifest file
appears first.

With these changes the jars actually work properly.

Thanks to Roel for reporting these issues to me privately and assisting
in debugging all of this!

~~ Ricardo