{% assign rawcats = "" %}
{% for post in site.posts %}
  {% assign tcats = post.categories | join:'|' | append:'|' %}
  {% assign rawcats = rawcats | append:tcats %}
{% endfor %}
{% assign rawcats = rawcats | split:'|' | sort %}

{% assign site.categories = "" %}
{% for category in rawcats %}
  {% if category != "" %}
    {% if categories == "" %}
      {% assign categories = category | split:'|' %}
    {% endif %}
    {% unless categories contains category %}
      {% assign categories = categories | join:'|' | append:'|' | append:category | split:'|' %}
    {% endunless %}
  {% endif %}
{% endfor %}