Resolve warning with newer macOS C++ compiler

This addresses issue 511 by disabling chained fixups, a new feature
in XCode 13 (macOS 11?) that is now on by default in XCode 14 (macOS 12)
and which may not work with undefined references when creating shared
objects (according to a warning).

platform.sh

@@ -60,7 +60,14 @@ fi
 if [ "$1" = "c++_shared_flags" ] ; then
     if [ "${OSTYPE}" = "Darwin" ]
     then
-	echo "-dynamiclib -Wl,-undefined,dynamic_lookup"
+	# macOS 12 (or XCode 14) requires an additional flag
+	SWVER=`sw_vers -productVersion`
+	SWVERMAJOR=$(echo ${SWVER} | cut -d'.' -f1)
+	if [ "$SWVERMAJOR" -ge 12 ] ; then
+	    echo "-dynamiclib -Wl,-undefined,dynamic_lookup -Wl,-no_fixup_chains"
+	else
+	    echo "-dynamiclib -Wl,-undefined,dynamic_lookup"
+	fi
     else
 	echo "-shared"
     fi