gh-120291: Fix a bashism in python-config.sh.in (#120292)

gh-120291: Fix bashisms in python-config.sh.in

Replace the use of bash-specific `[[ ... ]]` with POSIX-compliant
`[ ... ]` to make the `python-config` shell script work with non-bash
shells again.  While at it, use `local` in a safer way, since it is
not in POSIX either (though universally supported).

Fixes #120291