Undeclared identifiers

For our company's purposes, when we build the toolkit (version 13) we make a separate copy of the project file (ToolkitProShared_vc90.vcproj) and modify it slightly (as far as I know just added a custom define). I also had to add the following include to stdafx.h:
#ifdef _HEIXT
#pragma message( "Including Hostboot.h" )
#include "Hostboot.h"
#endif

_HEIXT is the custom define referred to previously.

Here are the contents of Hostboot.h:

----------------------------------------------------------------------------
#pragma once

// Modify the following defines if you have to target an OS before the ones
// specified in the following code. See MSDN for the latest information
// about corresponding values for different operating systems.
#ifndef WINVER
#define WINVER 0x0501            // XP, Server 2003, Vista and later
#endif

#ifndef _WIN32_WINNT
#define _WIN32_WINNT 0x0501    // XP, Server 2003, Vista and later
#endif

#ifndef _WIN32_WINDOWS
#define _WIN32_WINDOWS 0x0501    // XP, Server 2003, Vista and later
#endif

#ifndef _WIN32_IE
#define _WIN32_IE 0x0600        // Internet Explorer 6.0 or later.
#endif

#ifndef _hSTR2
#define _hSTR(x)    #x
#define _hSTR2(x)    _hSTR(x)
#endif
#pragma message("WINVER=" _hSTR2(WINVER) "  _WIN32_WINNT=" _hSTR2(_WIN32_WINNT) "  _WIN32_WINDOWS=" _hSTR2(_WIN32_WINDOWS) "  _WIN32_IE=" _hSTR2(_WIN32_IE))

#pragma warning( disable : 4996 )
----------------------------------------------------------------------------

This results in build errors like these:

Error    1    error C2065: 'MP_POPUPBACKGROUND' : undeclared identifier    c:\heikits\xtp1300\source\skinframework\xtpskinobjectmenu.cpp    824
Error    3    error C2065: 'MP_POPUPBACKGROUND' : undeclared identifier    c:\heikits\xtp1300\source\skinframework\xtpskinobjectmenu.cpp    1012
Error    17    error C2065: 'MP_POPUPBACKGROUND' : undeclared identifier    c:\heikits\xtp1300\source\commandbars\xtpnativexptheme.cpp    82
Error    23    error C2065: 'MP_POPUPBACKGROUND' : undeclared identifier    c:\heikits\xtp1300\source\commandbars\xtpnativexptheme.cpp    244
Error    2    error C2065: 'MP_POPUPBORDERS' : undeclared identifier    c:\heikits\xtp1300\source\skinframework\xtpskinobjectmenu.cpp    1010
Error    22    error C2065: 'MP_POPUPBORDERS' : undeclared identifier    c:\heikits\xtp1300\source\commandbars\xtpnativexptheme.cpp    241
Error    9    error C2065: 'MP_POPUPCHECK' : undeclared identifier    c:\heikits\xtp1300\source\skinframework\xtpskinobjectmenu.cpp    1197
Error    20    error C2065: 'MP_POPUPCHECK' : undeclared identifier    c:\heikits\xtp1300\source\commandbars\xtpnativexptheme.cpp    200
Error    21    error C2065: 'MP_POPUPCHECK' : undeclared identifier    c:\heikits\xtp1300\source\commandbars\xtpnativexptheme.cpp    212

There are quite a few more, but they're all undeclared identifier errors for MP_* defines in those two files. I realize that including our file is overriding the WINVER defines in XTPDLLExports.h and that, somehow, that's creating the problem. What's perplexing though is that I can't find these MP_* defines ANYWHERE I've looked. I searched in the XTP Toolkit directories, the mfc source directories, and the Microsoft SDK directory. Nothing. Even Google seems stumped on this.

If anyone knows where these defines exist and/or why they are gone when the WINVER changes from 0x0400 to 0x0501, please let me know.

Thanks.