Recognize macOS 11 and later and simplify.
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=44605
--- configure.orig	2009-03-03 17:38:38.000000000 -0600
+++ configure	2024-01-30 17:41:56.000000000 -0600
@@ -9554,15 +9554,10 @@
     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 ${wl}dynamic_lookup' ;;
       esac
     ;;
@@ -10717,18 +10712,14 @@
          allow_undefined_flag='${wl}-undefined ${wl}suppress'
          ;;
        *) # Darwin 1.3 on
-         if test -z ${MACOSX_DEPLOYMENT_TARGET} ; then
-           allow_undefined_flag='${wl}-flat_namespace ${wl}-undefined ${wl}suppress'
-         else
-           case ${MACOSX_DEPLOYMENT_TARGET} in
-             10.[012])
+           case $MACOSX_DEPLOYMENT_TARGET,$host in
+             10.[012],*|,*powerpc*-darwin[5-8]*)
                allow_undefined_flag='${wl}-flat_namespace ${wl}-undefined ${wl}suppress'
                ;;
-             10.*)
+             *)
                allow_undefined_flag='${wl}-undefined ${wl}dynamic_lookup'
                ;;
            esac
-         fi
          ;;
       esac
       archive_cmds_need_lc=no
@@ -17397,18 +17388,14 @@
          allow_undefined_flag_F77='${wl}-undefined ${wl}suppress'
          ;;
        *) # Darwin 1.3 on
-         if test -z ${MACOSX_DEPLOYMENT_TARGET} ; then
-           allow_undefined_flag_F77='${wl}-flat_namespace ${wl}-undefined ${wl}suppress'
-         else
-           case ${MACOSX_DEPLOYMENT_TARGET} in
-             10.[012])
+           case $MACOSX_DEPLOYMENT_TARGET,$host in
+             10.[012],*|,*powerpc*-darwin[5-8]*)
                allow_undefined_flag_F77='${wl}-flat_namespace ${wl}-undefined ${wl}suppress'
                ;;
-             10.*)
+             *)
                allow_undefined_flag_F77='${wl}-undefined ${wl}dynamic_lookup'
                ;;
            esac
-         fi
          ;;
       esac
       archive_cmds_need_lc_F77=no
@@ -20026,18 +20013,14 @@
          allow_undefined_flag_GCJ='${wl}-undefined ${wl}suppress'
          ;;
        *) # Darwin 1.3 on
-         if test -z ${MACOSX_DEPLOYMENT_TARGET} ; then
-           allow_undefined_flag_GCJ='${wl}-flat_namespace ${wl}-undefined ${wl}suppress'
-         else
-           case ${MACOSX_DEPLOYMENT_TARGET} in
-             10.[012])
+           case $MACOSX_DEPLOYMENT_TARGET,$host in
+             10.[012],*|,*powerpc*-darwin[5-8]*)
                allow_undefined_flag_GCJ='${wl}-flat_namespace ${wl}-undefined ${wl}suppress'
                ;;
-             10.*)
+             *)
                allow_undefined_flag_GCJ='${wl}-undefined ${wl}dynamic_lookup'
                ;;
            esac
-         fi
          ;;
       esac
       archive_cmds_need_lc_GCJ=no