configure wrongly warns about non-POSIX shell

Summary

configure always warns about the shell being non-POSIX, even when it is.

BIND version used

Bug in 9.13.4,5, not in 9.12.3

Steps to reproduce

Prepare the installation source by unpacking a source archive or otherwise.

In the top-level directory of the source, run ./configure [options] from eg dash as /bin/sh.

What is the current bug behavior?

A warning that configure will continue with the shell even though it is non-POSIX is interleaved in the command output.

What is the expected correct behavior?

configure should recognise /bin/sh (or possibly some other shell in its search path) as being POSIX enough and not display a warning.

Possible fixes

The sufficient condition is based on $(expr ...) and $((...)) being supported and giving the same result.

The script fragment ca. line 11680 in configure that assesses the available shells against this condition uses a loop that fails to exit. Consequently it always finds the guard value "no".

After line 11702, when the sufficiently POSIX shell has been found and stashed in a variable, append a line

break