selfupdate should use parallel build

[ryandesign (Ryan Carsten Schmidt)]

Is there a good reason why selfupdate doesn't build MacPorts base in parallel?

[ryandesign (Ryan Carsten Schmidt)]

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.

[jmroot (Joshua Root)]

In b98b7f18954db8ae630730b1429878dae07f3002/macports-base (master):

Obey buildmakejobs in selfupdate

Closes: #56435