Hello Erik, all,

> Note that this has a straightforward work-around -- pass the respective
> options manually. This short before a release, I recommend going with a
> work-around instead of modifying Cactus's internal mechanism, which might
> break things.
Context: this affects the auto-build MPI library only, not the use of
an existing MPI library found on a cluster or even the one found on
ubuntu, debian, OSX etc. As far as I can see there is thus no way of
"manually" specifying extra libraries (in an option list)

since this
only happens in exactly the case where the option list says "build
yourself" or the auto-detection fails.

Frank already gave the answer as to where to modify things (it
literally is) in his email:

$ENV{MPI_LIBS} .= " rt util" if($^O == "linux");

and as said rt and util as part of libc6 anyway so they are always
present.

Yours,
Roland

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