[MacPorts] #56435: selfupdate should use parallel build

[MacPorts]

#56435: selfupdate should use parallel build
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  Reporter:  ryandesign   |      Owner:  (none)
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  base         |    Version:  2.4.99
Resolution:               |   Keywords:
      Port:               |
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Comment (by ryandesign):

 Assuming there isn't a good reason, we should make it build in parallel.
 Base has grown over the years, with bundled copies of other libraries, and
 now that Macs have more processor cores, parallel building can save a bit
 of time.

 How should it be done? I hoped I could reuse the existing functionality in
 `portbuild::build_getjobsarg` but I assume most of that procedure (except
 the parts that check `use_parallel_build` and `build.cmd`), and all of
 `portbuild::build_getjobs`, should be moved into a file that's not meant
 to be specific to the port build phase. If so, where should it go? Into
 `macports::`? The tests will then need to be updated too, since they test
 those procedures.

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Ticket URL: <https://trac.macports.org/ticket/56435#comment:1>
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