Recognize macOS 11 and later and simplify.
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=44605
--- configure.orig	2006-02-21 02:03:22.000000000 -0600
+++ configure	2021-07-06 17:51:33.000000000 -0500
@@ -8323,18 +8323,14 @@
          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 ${wl}dynamic_lookup'
                ;;
            esac
-         fi
          ;;
       esac
       archive_cmds_need_lc=no
@@ -11302,18 +11298,14 @@
          allow_undefined_flag_CXX='${wl}-undefined ${wl}suppress'
          ;;
        *) # Darwin 1.3 on
-         if test -z ${MACOSX_DEPLOYMENT_TARGET} ; then
-           allow_undefined_flag_CXX='${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_CXX='${wl}-flat_namespace ${wl}-undefined ${wl}suppress'
                ;;
-             10.*)
+             *)
                allow_undefined_flag_CXX='${wl}-undefined ${wl}dynamic_lookup'
                ;;
            esac
-         fi
          ;;
         esac
       archive_cmds_need_lc_CXX=no
@@ -15369,18 +15361,14 @@
          allow_undefined_flag_F77='${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 ${wl}dynamic_lookup'
                ;;
            esac
-         fi
          ;;
       esac
       archive_cmds_need_lc_F77=no
@@ -17839,18 +17827,14 @@
          allow_undefined_flag_GCJ='${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