The postgres module queries PostgreSQL from nginx itself, with no
application in between: a location names an upstream and a query, and the
rows come back as text, as JSON, or as nothing but a status code.  Values
from the request go into the query escaped, and the status code can be
chosen by how many rows came back.

This port only installs the module sources.  The module itself is built by
the nginx ports when their POSTGRES option is enabled.