Auto 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.

Author: bors

configure

@@ -697,15 +697,17 @@ probe CFG_ADB        adb
 
 if [ ! -z "$CFG_PANDOC" ]
 then
-    PV_MAJOR_MINOR=$(pandoc --version | grep '^pandoc\(.exe\)\? ' |
-        # extract the first 2 version fields, ignore everything else
-        sed 's/pandoc\(.exe\)\? \([0-9]*\)\.\([0-9]*\).*/\2 \3/')
+    PV_MAJOR_MINOR=$(pandoc --version | grep '^pandoc' |
+        # Extract "MAJOR MINOR" from Pandoc's version number
+        sed -E 's/pandoc(.exe)? ([0-9]+)\.([0-9]+).*/\2 \3/')
 
     MIN_PV_MAJOR="1"
     MIN_PV_MINOR="9"
+
     # these patterns are shell globs, *not* regexps
     PV_MAJOR=${PV_MAJOR_MINOR% *}
     PV_MINOR=${PV_MAJOR_MINOR#* }
+
     if [ "$PV_MAJOR" -lt "$MIN_PV_MAJOR" ] || [ "$PV_MINOR" -lt "$MIN_PV_MINOR" ]
     then
         step_msg "pandoc $PV_MAJOR.$PV_MINOR is too old. Need at least $MIN_PV_MAJOR.$MIN_PV_MINOR. Disabling"