*Note: This change is included in concrete5.4.2a2+

First copy /concrete/elements/header_required.php to /elements.

Open the file and find:

<meta name="generator" content="concrete5 - <?=APP_VERSION ?>" />

or

<meta name="generator" content="concrete5 - <?php echo APP_VERSION ?>" />

Under it add:

<?php if($c->getCollectionAttributeValue('exclude_search_index')) { ?>
    <meta name="robots" contents="noindex" />
<?php } ?>

If the Exclude from Search Index attribute is checked it will add that to the page, and it will prevent search robots from indexing that page.