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/9353
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>
diff --git a/configure.ac b/configure.ac
index 28d4291..ef6bf92 100644
--- a/configure.ac
+++ b/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