Darwin : Update libtool and dependencies for Darwin20 [PR97865]

The change in major version (and the increment from Darwin19 to 20)
caused libtool tests to fail which resulted in incorrect build settings
for shared libraries.

	PR target/97865
	* libtool.m4: Update handling of Darwin platform link flags
	for Darwin20.

libtool.m4

@@ -994,22 +994,24 @@ _LT_EOF
         rm -f conftest.err libconftest.a conftest conftest.c
         rm -rf conftest.dSYM
     ])
-    case $host_os in
-    rhapsody* | darwin1.[[012]])
+    # Allow for Darwin 4-7 (macOS 10.0-10.3) although these are not expect to
+    # build without first building modern cctools / linker.
+    case $host_cpu-$host_os in
+    *-rhapsody* | *-darwin1.[[012]])
       _lt_dar_allow_undefined='${wl}-undefined ${wl}suppress' ;;
-    darwin1.*)
+    *-darwin1.*)
       _lt_dar_allow_undefined='${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' ;;
+    *-darwin*)
+      # darwin 5.x (macOS 10.1) onwards we only need to adjust when the
+      # deployment target is forced to an earlier version.
+      case ${MACOSX_DEPLOYMENT_TARGET-UNSET},$host in
+	UNSET,*-darwin[[89]]*|UNSET,*-darwin[[12]][[0123456789]]*)
+	  ;;
 	10.[[012]][[,.]]*)
-	  _lt_dar_allow_undefined='${wl}-flat_namespace ${wl}-undefined ${wl}suppress' ;;
-	10.*)
-	  _lt_dar_allow_undefined='${wl}-undefined ${wl}dynamic_lookup' ;;
+	  _lt_dar_allow_undefined='${wl}-flat_namespace ${wl}-undefined ${wl}suppress'
+	  ;;
+	*)
+	  ;;
      esac
     ;;
   esac
@@ -1033,7 +1035,7 @@ _LT_EOF
 
 # _LT_DARWIN_LINKER_FEATURES
 # --------------------------
-# Checks for linker and compiler features on darwin
+# Checks for linker and compiler features on Darwin / macOS / iOS
 m4_defun([_LT_DARWIN_LINKER_FEATURES],
 [
   m4_require([_LT_REQUIRED_DARWIN_CHECKS])