So even though I was copying System.Net.Http .NET 4.0 via the Nuget package that is added when installed the DotNetOpenAuth packages, MsBuild on my build server was checking the DotNetOpenAuth.Core dependency and checking against .NET assemblies installed on the machine not what was being copied to the Bin. In my case:
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0
And there was no System.Net.Http assembly in my
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0
directory so I believe MsBuild attempted to look in the newly installed .NET 4.5 assemblies, on my machine..\Assemblies\Microsoft\Framework.NETFramework\v4.5 where there is a System.Net.Http.dll and that is why, instead of getting an outright error that System.Net.Http.dll for .NET 4.0 targets does not exist, I got a warning about DotNetOpenAuth.Core.dll's dependency on System.Net.Http, which can be found but is not for the 4.0 framework as the project was targeting.
I simply copied System.Net.Http .NET 4.0 DLL from the package to
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0
on my build server and MsBuild was able to compare to the correct version System.Net.Htpp for 4.0 .NET framework dependency check.