[MacPorts] #48940: go: update to 1.5.1

[MacPorts]

#48940: go: update to 1.5.1
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  Reporter:  lutz.horn@…  |      Owner:  ciserlohn@…
      Type:  update       |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  ports        |    Version:
Resolution:               |   Keywords:
      Port:  go           |
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Comment (by nicholas@…):

 As of version 1.5, the Go toolchain is written in Go.  This means that in
 order to build Go 1.5, one must either already have Go 1.4.x installed or
 use the [https://golang.org/doc/install/gccgo gccgo] compiler.

 Should the Portfile build Go 1.4 as a prerequisite to building Go 1.5?  Or
 would it be preferable to add Go support to
 [browser:trunk/dports/lang/gcc5/Portfile gcc5] (possibly as a variant) and
 specify it as a build dependency?

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Ticket URL: <https://trac.macports.org/ticket/48940#comment:3>
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