auto/config: use syntax compatible with "set -e" (refs: #15107).
This script has "set -e" already. I think that's currently ignored due to the way live-build runs it, but let's not create confusion by writing code in there that will break whenever "set -e" becomes effective. Besides, we try to use "set -e" everywhere so let's write code that's compatible with it, which can be useful e.g. if we copy'n'paste code around.
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