[MacPorts] #69404: shouldn't xz-bootstrap usage be controlled through "base"?

[MacPorts]

#69404: shouldn't xz-bootstrap usage be controlled through "base"?
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 Reporter:  RJVB         |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  base         |    Version:
 Keywords:               |       Port:
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 I just notice that port:libunwind extract_depends on port:xz-bootstrap, in
 order to ''minimise dependency tree''.

 As it turns out it does the opposite for people who build the port from
 source when they already have port:xz installed... If port:xz-bootstrap is
 indeed only used by port:clang-3.4 it's a bit silly to impose the
 installation on systems where that OLD compiler is devoid of (almost) all
 interest...

 Of course each individual port could just check `[file exists
 ${prefix}/bin/xz]` to determine whether a depends_extract is required, but
 isn't that something that should be part of the central `use_xz` codepaths
 in "base"?
 AFAICT this approach should be perfectly safe without unwanted side-
 effects since it's relevant for building from source only.

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Ticket URL: <https://trac.macports.org/ticket/69404>
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