Fortran calling C++ stdcall DLL

Why can I link my Intel Fortran  dll (sum_if.dll)  to my Fortran (sum_test)  but not my C++ one (sum_cpp.dll)?     All are accepted as STDCAL DLLs by other software e.g. Excel VBA (in the attached .zip)

Fortran Linking to sum_if  works with any of:

1. Linker | Input |  Additional Dependecies= sum_if.lib
2. Adding.lib to project (e.g. sum_if.lib)
3. Project Dependencies = sum_if

But for sum_cpp the dll is not found or accepted   i.e.

sum_test.obj : error LNK2019: unresolved external symbol __imp_sum referenced in function _MAIN__

Dependency Walker  shows function=sum on both. So hopefully it is not a "Decoration" issue?

I have attached the VS Community 2013 Solution, as I hope/ expect  the answer is hidden there. 

1. VS 2013 Solution= sumdll  Projects: = Win32, 

All sharing same ..\Output\Release and Debug directories.  The excel “double check”  lives in Debug. This helps simplify and clarify "finding" the DLL (I wanted to avoid full paths) 

1.1 Sum_if.dll contains:

function sum(a,b)
IMPLICIT NONE
  ! Expose subroutine ExcelDLL to users of this DLL
  !DEC$ATTRIBUTES STDCALL, DLLEXPORT, ALIAS: 'sum'	:: sum
  !DEC$ATTRIBUTES Value :: a,b
  ! Variables
  integer*4 a,b,sum
  ! Body of ExcelDLL
  sum=a+b
end function sum

1.2 Sum_cpp.dll 

//  Sum_cpp.cpp contains:
int _stdcall sum(int x, int y){
	return x+y;
}

// Sum_cpp.def contains:
LIBRARY	"sum_cpp""
	EXPORTS sum @

1.3 Sum_test

   Interface
      function sum(a,b)
        IMPLICIT NONE
        ! Expose subroutine ExcelDLL to users of this DLL
        !DEC$ ATTRIBUTES DLLIMPORT, ALIAS:'sum'	:: sum
        !DEC$ATTRIBUTES Value :: a,b
        ! Variables
        INTEGER*4 a,b,sum
      end function sum
    End Interface

    INTEGER*4  :: a_i4b, b_i4b, c_i4b,d_i4b, I, j
    print *, 'add integers'
    a_i4b=7
    b_i4b=43
    c_i4b=sum(a_i4b,b_i4b)
    write(*,'(I10,A,I10,A,I10)')  a_i4b," + ", b_i4b," = ",c_i4b
    end program vf_dll_test

2. Excel VBA Declares

' all work fine:' C++
Declare Function sumcpp Lib "sum_cpp.dll" Alias "sum" (ByVal i As Long, ByVal j As Long) As Long' Intel Fortran 10.1
Declare Function sumif Lib "sum_if.dll" Alias "sum" (ByVal a As Long, ByVal b As Long) As Long

Peter