[External] : configure shell game

[Nico Williams]

Oh, and also the set of warnings that a POSIX shell is allowed to emit
might not be specified by POSIX.  Looking at
https://pubs.opengroup.org/onlinepubs/9799919799/utilities/sh.html

| Except as otherwise stated (by the descriptions of any invoked
| utilities or in interactive mode), standard error shall be used only
| for diagnostic messages.

Warnings about `` are -IMO- not diagnostic messages so, I think you're
quite right that this is a bug in Solaris.

Nico
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