[strace/strace] configure: check AR variable for 'ar' tool (#137)

[codecov[bot]]

# [Codecov](https://codecov.io/gh/strace/strace/pull/137?src=pr&el=h1) Report
> Merging [#137](https://codecov.io/gh/strace/strace/pull/137?src=pr&el=desc) into [master](https://codecov.io/gh/strace/strace/commit/ddd0b84cbf812914da5d326291fa3965ac023386&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/strace/strace/pull/137/graphs/tree.svg?width=650&height=150&src=pr&token=Uoc5vM53zG)](https://codecov.io/gh/strace/strace/pull/137?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master     #137   +/-   ##
=======================================
  Coverage   86.06%   86.06%           
=======================================
  Files         260      260           
  Lines       17824    17824           
=======================================
  Hits        15341    15341           
  Misses       2483     2483           
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/strace/strace/pull/137?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/strace/strace/pull/137?src=pr&el=footer). Last update [ddd0b84...81e11a9](https://codecov.io/gh/strace/strace/pull/137?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


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