Accept Coq version 8.6.1 as supported

8.6.1 works just fine with the current CompCert.

configure

@@ -470,7 +470,7 @@ missingtools=false
 echo "Testing Coq... " | tr -d '\n'
 coq_ver=$(${COQBIN}coqc -v 2>/dev/null | sed -n -e 's/The Coq Proof Assistant, version \([^ ]*\).*$/\1/p')
 case "$coq_ver" in
-  8.6)
+  8.6|8.6.1)
         echo "version $coq_ver -- good!";;
   ?*)
         echo "version $coq_ver -- UNSUPPORTED"