Include unmanaged DLL when deploying ASP.NET application
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I have an ASP.NET web application that depends on a DLL with unmanaged code.  If I try to "add" this DLL as a reference for the project, I get an error saying 

    A reference to 'c:\path\to\lib' could not be added. 
    Please make sure the file is accessible, and that it is a valid 
    assembly or COM component

However, if it is not part of the project-references list, it does not get included when doing a Build->Publish (or when publishing from, say, TeamCity).  How do I get this DLL to be automatically included when deploying/publishing?