Smart Map - Outputting locations from a Matrix field for all entries within a single section
Great question! While seemingly tricky, the solution to this should be relatively simple...
Per the documentation, the locations parameter can come in a variety of different forms. But the one we're looking for here is this one...

As an array of elements

Collecting Matrix blocks from across different entries can be complicated, but I think you have (fortunately) described a fairly straightforward use-case. We'll need to do the following:

Loop through relevant entries.
Build an array of Matrix blocks.
Use that array to render a map.


{# Initialize locations array #}
{% set locationsArray = [] %}

{# Loop through all relevant entries #}
{% for entry in craft.entries.section('buildings').find() %}

    {# Get all relevant Matrix blocks #}
    {% set matrixBlocks = entry.buildingContent.type('howToGetThere').find() %}

    {# Append blocks to locations array #}
    {% set locationsArray = locationsArray|merge(matrixBlocks) %}

{% endfor %}

{# Render map #}
{{ craft.smartMap.map(locationsArray, options) }}