Windows namespace resolution issue in header file

Question

I'm trying to incorporate the Rx Extensions for C++ into my application and I'm running into a fundamental problem of referencing the Windows namespace in the provided header files. I created a new project as a Windows Store DLL and when I include the rx header files I get the error:

rx-winrt.hpp(15): error C2653: 'Windows' : is not a class or namespace name 

This is the one of the sections of code where the error occurs.

namespace rxcpp { namespace winrt {
    namespace wf = Windows::Foundation;
    namespace wuicore = Windows::UI::Core;
    namespace wuixaml = Windows::UI::Xaml;
    namespace wuictrls = Windows::UI::Xaml::Controls;

From what I can tell all the proper headers are included. The only difference between this project and another support library I wrote is that the problem project defines _WINDLL. The other project that is working defines _WINRT_DLL. I tried changing the in the problem project but that did not help. Any help on this problem would be greatly appreciated.