Lmk if I should add tests and/or changes to the change log

@krobelus What would you like me to do for casting flags? I made it more concise but now clippy is failing. I think a clippy ignore is probably a bad solution here. From my understanding, the only real solution is going to be the original version (cfg_if) with some duplication.

We use #[allow(clippy::unnecessary_cast)] for several such situations. If we want to avoid unsafe casts, I think we'd need a separate code path doing something like https://doc.rust-lang.org/std/primitive.i32.html#method.try_from-4 for the configurations where a type conversion is needed.

I think we should just remove the type annotation on flags. Leaves us with just:

let mut flags = 0;
flags |= libc::POSIX_SPAWN_SETSIGDEF;
flags |= libc::POSIX_SPAWN_SETSIGMASK;
if desired_pgid.is_some() {
    flags |= libc::POSIX_SPAWN_SETPGROUP;
}
attr.set_flags(flags.try_into().expect("Flags should fit in c_short"))?;

Added. Lmk if you want any changes. Otherwise, should be good to merge.