portal-ext.properties:

dl.store.impl=com.liferay.portlet.documentlibrary.store.CMISStore
dl.store.cmis.credentials.username=none
dl.store.cmis.credentials.password=none
dl.store.cmis.repository.url=http://localhost:8080/alfresco/service/api/cmis
dl.store.cmis.system.root.dir=Liferay Home

Source file location: ./portal-impl/src/com/liferay/portlet/documentlibrary/store/CMISStore.java.