Rollup merge of #22474 - iKevinY:pandoc-version-check, r=brson
Rollup merge of #22474 - iKevinY:pandoc-version-check, r=brson

 Executing `configure` seems to create the following error due to how the script [parses Pandoc's version](https://github.com/rust-lang/rust/blob/master/configure#L705):

```text
./configure: line 705: [: pandoc: integer expression expected
./configure: line 705: [: 1.12.4.2: integer expression expected
```

This issue seems to stem from a discrepancy between BSD and GNU versions of sed. This patch changes the sed command to use an extended regex, which works with both flavours of sed.

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