This allows users to configure the Mail Transfert Agent (MTA) of their
choice.
* configure.ac: Add '--with-sendmail' option.
(SENDMAIL): Default to 'sendmail -t'.
* NEWS: Announce it.
* src/mcron/redirect.scm (with-mail-out): Assume the MTA is reading the
message for recipients.
* build-aux/guix.scm: Remove 'which' from the native-inputs.