Add an option to set the mtime of entries in a tarball so that one can
create a tarball with a consistent content hash (e.g. MD5) for a given
set of files regardless of their current timestamps on disk. This will
be useful for submission of tarballs to CDash, which tracks content
hashes to avoid duplication.
Inspired-by: Bill Hoffman <bill.hoffman@kitware.com>
Brad King