Use "parallel" config setting and 4 as defaults

[radarhere]

Suggestion for python-pillow#8990

https://pillow.readthedocs.io/en/stable/installation/building-from-source.html#build-options

Config setting: -C parallel=n. Can also be given with environment variable: MAX_CONCURRENCY=n. Pillow can use multiprocessing to build the extension. Setting -C parallel=n sets the number of CPUs to use to n, or can disable parallel building by using a setting of 1. By default, it uses 4 CPUs, or if 4 are not available, as many as are present.

This PR allows -C parallel=n to be used, and 4 otherwise.

[radarhere]

Cherry-picked into the PR.