ENH: Added -j auto for build cmd

[ganesh-k13]

Changes

• Added a -j auto for spin build

Testing

(np-test) ➜  numpy git:(main) spin build -j auto
Building with 12 parallel jobs
$ /<>/os/np-test/bin/python3 vendored-meson/meson/meson.py compile -j 12 -C build
INFO: autodetecting backend as ninja
...

We can speed up CI jobs for all NumPy workflows post this using -j auto

[stefanv]

I think Meson has this built-in, if you set -j 0? But, it also seems to be on by default (I just checked, and NumPy compilation uses all cores).

[ganesh-k13]

Oh, my laptop was always using a single core, not sure why. Maybe an issue from my end, let me take a look.

[rgommers]

This shouldn't be needed indeed. By default, ninja will use 2*n_cpu + 2 build processes (e.g., if your machine has 12 physical cores, it'll use max 26 processes). It isn't even possible to change that with an environment variable, only by explicitly passing -j1. So please look at how ninja gets invoked on your machine.