BrokeredPointOfService requires POS for .NET 1.14. As long as you have the POS for .NET 1.14 assemblies installed on your machine, the BrokeredPointOfService solution should build as-is in your cloned repository.
The BrokeredPointOfService library requires a system level environment variable named BrokeredPointOfService.
For development, this should point to the output directory of BrokeredPointOfServiceProxyStub project.
For a full information on building, testing and extending BrokeredPointOfService, see http://blogs.msdn.com/b/apulliam/archive/2014/11/01/introducing-brokeredpointofservice-brokered-windows-runtime-component-wrapper-library-for-pos-for-net-1-14.aspx