Correctly report backtrace support when `configure` finishes

Tony Finch requested to merge fanf/bind9:u/fanf2/configure-backtrace into master

I was puzzled that configure was reporting that I did not have backtrace support enabled. But this turned out to be because configure's feature report was using the wrong variable name!

Here's a fix. I have not committed the results of autoreconf because that resulted in a lot of irrelevant churn due to mismatched versions of autotools, so it's probably best if you do that.

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