Dear maintainer,

a common way to use `make` on Continuous Integration systems like Jenkins, and some other batch processing systems, is to wrap it in the `unshare` program from `util-linux`, enabling process namespaces.

Example:

    unshare -Upf --kill-child -- make runMyCI

The reason for doing so is so that when you kill `unshare`, it ensures that all recursive child processes are reliably killed, and no zombie processes are left, no matter if they misbehave (e.g. backgrounding by double-forking) due to bugs or bad practices (which is especially annoying for cron jobs, CI servers, or other types of setup where process zombies/reparenting needs to be avoided).

This worked great up to including gnumake 4.2.1, but it stopped working with 4.3

A workaround is to configure gnumake with `--disable-posix-spawn`.

Concretely, the observed error is:

    unshare -Ufp --kill-child make
    make: sh: Invalid argument
    make: * [Makefile:6: help] Error 127

`strace -fy` on the above command reveals that this `Invalid argument` comes from the `setresuid()` syscall, which is invoked by the first forked child of the `make` main process:

    [pid  6545] getuid()                    = 65534
    [pid  6545] setresuid(-1, 65534, -1)    = -1 EINVAL (Invalid argument)

It would be great to know whether make 4.3 could be made work under `unshare` in general, or if not, if the switch to disable poxix-spawn could be made available at run-time, so that users can use continue to use their distribution packages while still having guaranteed child killing via `unshare`.

Thank you!

nh2