Build failure on macOS 10.15

[Russ Allbery]

Julien ÉLIE <julien at trigofacile.com> writes:

> Suggested fix:

I've applied the relevant part of this to rra-c-util as well, with a minor
tweak to the comment and to the configure output (because so far as I can
tell Autoconf doesn't promise to always set $LD).

Thanks!

-- 
Russ Allbery (eagle at eyrie.org)             <https://www.eyrie.org/~eagle/>

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