Visual Studio Conversion: Use of macros in file path no longer supported

Hi.  When upgrading projects from VS2008 to VS2013, the conversion reports the following:

   foo.vcproj: Use of macros in file path no longer supported: '$(IntDir)foo.cpp'. This file has been removed during conversion.

We generate c/cpp files with custom build steps and place the generated code in $(IntDir) to be compiled.  This works in VS2008, and VS2012.  However, when I tried to convert a project from VS2008 to VS2013 the conversion tool deleted the files from the vcxproj.

For example, here is a VS2008 project listing that didn't convert.

   <File
    RelativePath="$(IntDir)\foo.cpp"
    >
   </File>

Is this purely an issue with the conversion tool, or are macros truly not supported when listing files? 

Thanks,

Jon