Right now I don't, because the project takes ~30 seconds to build serially... There are a few options:

1. Put the Ninja support in the tarball (which is trivial, but I guess I was paranoid about the extra dependency for a low level tool like shell). I think Ninja is supported everywhere now, including on Windows and BSDs. It's easier to build that GNU make itself, etc.

2. The last time I looked, the toybox project had some sort of "parallel job server" type thing in shell, to avoid GNU make.

3. Maybe do something simpler with "xargs -P" (non POSIX)

I'd probably go for the first option just because it's the least custom code to solve this particular problem. If the build was slower, it might be different ... although if speed is really an issue, then Ninja is the best option out of all of these, AND GNU make!

----

edit: I should also note that in practice I think many users and especially distros do NOT pass -j $N to GNU make because it's not reliable. If upstream didn't test with it, there can be bugs that result in incorrect builds. So distros often just use serial builds.

On the other hand, paralllel builds with Ninja are the default, so upstream will have tested in that mode.