macOS Big Sur is 11.0, Darwin 20, after a long line of 10.X releases.

--- configure
+++ configure
@@ -8121,7 +8121,7 @@
 	  _lt_dar_allow_undefined='${wl}-undefined ${wl}dynamic_lookup' ;;
 	10.[012][,.]*)
 	  _lt_dar_allow_undefined='${wl}-flat_namespace ${wl}-undefined ${wl}suppress' ;;
-	10.*)
+	10.*|11.*)
 	  _lt_dar_allow_undefined='${wl}-undefined ${wl}dynamic_lookup' ;;
       esac
     ;;