[MacPorts] #47814: gnutar: update to 1.28

[MacPorts]

#47814: gnutar: update to 1.28
--------------------------+--------------------
 Reporter:  ryandesign@…  |      Owner:  mww@…
     Type:  update        |     Status:  new
 Priority:  Normal        |  Milestone:
Component:  ports         |    Version:  2.3.99
 Keywords:  haspatch      |       Port:  gnutar
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 gnutar 1.28 is available. This patch updates the port to this version. If
 you have no objection I'm happy to commit it.

 gnutar 1.28 fails to build on OS X out of the box; this includes a patch
 from [http://lists.gnu.org/archive/html/bug-tar/2014-08/msg00001.html the
 mailing list] that fixes this.

 I also add the standard modeline; disable silent rules; and switch to
 using the xz distfile because it is smaller than the bz2 distfile.

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Ticket URL: <https://trac.macports.org/ticket/47814>
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