[m-rev.] for post-commit review: fix decision about parallel folds

[Peter Wang]

On Mon, 26 Aug 2024 00:00:10 +0200 "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> For review by anyone.
> 
> Zoltan.

> Fix the decision whether to use parallel folds.
> 
> compiler/make.build.m:
>     If the options say we should use parallel folds, but we cannot allocate
>     a job control structure, do not consider this as a sign of a problem
>     *building* the targets we are folding over, since we did not even
>     *attempt* to build those targets. Instead, in such cases, fall back
>     to folding over the targets sequentially.

I think the idea was that failing to set up the IPC for a parallel fold
is unexpected, so we should stop ASAP. But falling back to sequential
operation could be okay.

Peter