Recognize macOS 11 and later, remove inaccurate comment, and simplify.
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=44605
--- configure.orig	2010-11-25 13:19:01.000000000 -0600
+++ configure	2021-02-03 19:10:20.000000000 -0600
@@ -7381,15 +7381,10 @@
     darwin1.*)
      _lt_dar_allow_undefined='${wl}-flat_namespace ${wl}-undefined ${wl}suppress' ;;
     darwin*)
-      # 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]*)
+      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 ${wl}dynamic_lookup' ;;
       esac
     ;;
@@ -8544,7 +8539,7 @@
              10.[012])
                allow_undefined_flag='${wl}-flat_namespace ${wl}-undefined ${wl}suppress'
                ;;
-             10.*)
+             *)
                allow_undefined_flag='${wl}-undefined ${wl}dynamic_lookup'
                ;;
            esac
@@ -15208,7 +15203,7 @@
              10.[012])
                allow_undefined_flag_F77='${wl}-flat_namespace ${wl}-undefined ${wl}suppress'
                ;;
-             10.*)
+             *)
                allow_undefined_flag_F77='${wl}-undefined ${wl}dynamic_lookup'
                ;;
            esac
@@ -17842,7 +17837,7 @@
              10.[012])
                allow_undefined_flag_GCJ='${wl}-flat_namespace ${wl}-undefined ${wl}suppress'
                ;;
-             10.*)
+             *)
                allow_undefined_flag_GCJ='${wl}-undefined ${wl}dynamic_lookup'
                ;;
            esac