beautifulSA-jekyll/_includes/navbarlink.html

{% capture before %}{{ include.link[1] | split: "://" | first }}{% endcapture %}
{% capture after %}{{ include.link[1] | split: "://" | last }}{% endcapture %}
{% assign internal = true %}
{% if before != after %}
  {% if before == "http" or before == "https" %}
    {% assign internal = false %}
  {% endif %}
{% endif %}

{% if internal %}
  {% capture linkurl %}{{ include.link[1] | relative_url }}{% endcapture %}
{% else %}
  {% capture linkurl %}{{ include.link[1] }}{% endcapture %}
{% endif %}

<a href="{{ linkurl }}">{{ include.link[0] }}</a>