see https://github.com/mojodna/carto-posm-simple

project.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE Map[]>
<Map srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over" buffer-size="128" background-color="#ffffff">

<Parameters>
  <Parameter name="name"><![CDATA[POSM]]></Parameter>
  <Parameter name="attribution"><![CDATA[Data © OpenStreetMap Contributors]]></Parameter>
  <Parameter name="center">-75.525,39.158,12</Parameter>
  <Parameter name="format">png</Parameter>
  <Parameter name="minzoom">0</Parameter>
  <Parameter name="maxzoom">22</Parameter>
  <Parameter name="scale">1</Parameter>
  <Parameter name="metatile">4</Parameter>
</Parameters>


<Style name="highway" filter-mode="first">
  <Rule>
    <LineSymbolizer stroke-width="2" stroke="#666666" />
  </Rule>
</Style>
<Layer name="highway"
  status="on"
  srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
    <StyleName>highway</StyleName>
    <Datasource>
       <Parameter name="type"><![CDATA[postgis]]></Parameter>
       <Parameter name="dbname"><![CDATA[gis]]></Parameter>
       <Parameter name="geometry_field"><![CDATA[way]]></Parameter>
       <Parameter name="srid"><![CDATA[3857]]></Parameter>
       <Parameter name="extent"><![CDATA[-20037508.34,-20037508.34,20037508.34,20037508.34]]></Parameter>
       <Parameter name="srs"><![CDATA[+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over]]></Parameter>
       <Parameter name="table"><![CDATA[(
  SELECT
    *
  FROM planet_osm_roads
  WHERE way && !bbox!
) AS data
]]></Parameter>
    </Datasource>
  </Layer>

<Style name="building" filter-mode="first">
  <Rule>
    <LineSymbolizer stroke-width="2" stroke="#333333" />
    <PolygonSymbolizer fill="#f9f9f9" />
  </Rule>
</Style>
<Layer name="building"
  srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
    <StyleName>building</StyleName>
    <Datasource>
       <Parameter name="type"><![CDATA[postgis]]></Parameter>
       <Parameter name="dbname"><![CDATA[gis]]></Parameter>
       <Parameter name="geometry_field"><![CDATA[way]]></Parameter>
       <Parameter name="srid"><![CDATA[3857]]></Parameter>
       <Parameter name="extent"><![CDATA[-20037508.34,-20037508.34,20037508.34,20037508.34]]></Parameter>
       <Parameter name="srs"><![CDATA[+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over]]></Parameter>
       <Parameter name="table"><![CDATA[(
  SELECT
    *
  FROM planet_osm_polygon
  WHERE way && !bbox!
    AND building IS NOT NULL
) AS data
]]></Parameter>
    </Datasource>
  </Layer>

</Map>