gnu: rdup: Update to 1.1.15.

* gnu/packages/backup.scm (rdup): Update to 1.1.15.
[source]: Use new URI.  Remove snippet.
[native-inputs]: Add autoconf and automake.
[inputs]: Add mcrypt.
[arguments]: Add ‘bootstrap’ and ‘qualify-inputs’ phases.
Remove ‘remove-Werror’ phase.
[home-page]: Use apparent new (and HTTPS) home page.