use bundled (non-system) versions of libxml2 and libxslt for OS X <10.6

fixes fdo#78612 and fdo#77893

Change-Id: Iac36211095cc009a356f335a93a184ea5078214d
Reviewed-on: https://gerrit.libreoffice.org/9353Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>

configure.ac

@@ -7820,7 +7820,11 @@ if test "$with_system_libxml" = "auto"; then
         with_system_libxml="$with_system_libs"
         ;;
     *)
-        with_system_libxml=yes
+        if test "$_os" = "Darwin" -a $MAC_OS_X_VERSION_MAX_ALLOWED -lt 1060; then
+            with_system_libxml=no
+        else
+            with_system_libxml=yes
+        fi
         ;;
     esac
 fi