Recognize macOS 11 and later, remove inaccurate comment, and simplify.
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=44605
--- m4/libtool.m4.orig	2015-01-20 10:15:19.000000000 -0600
+++ m4/libtool.m4	2021-01-08 10:59:09.000000000 -0600
@@ -1067,16 +1067,11 @@
       _lt_dar_allow_undefined='$wl-undefined ${wl}suppress' ;;
     darwin1.*)
       _lt_dar_allow_undefined='$wl-flat_namespace $wl-undefined ${wl}suppress' ;;
-    darwin*) # darwin 5.x on
-      # if running on 10.5 or later, the deployment target defaults
-      # to the OS version, if on x86, and 10.4, the deployment
-      # target defaults to 10.4. Don't you love it?
-      case ${MACOSX_DEPLOYMENT_TARGET-10.0},$host in
-	10.0,*86*-darwin8*|10.0,*-darwin[[91]]*)
-	  _lt_dar_allow_undefined='$wl-undefined ${wl}dynamic_lookup' ;;
-	10.[[012]][[,.]]*)
+    darwin*)
+      case $MACOSX_DEPLOYMENT_TARGET,$host in
+	10.[[012]],*|,*powerpc*-darwin[[5-8]]*)
 	  _lt_dar_allow_undefined='$wl-flat_namespace $wl-undefined ${wl}suppress' ;;
-	10.*)
+	*)
 	  _lt_dar_allow_undefined='$wl-undefined ${wl}dynamic_lookup' ;;
       esac
     ;;