Compiling sources fails with 37 errors

[Twilight-Logic]

I'm trying to compile on Windows using Visual Studio 2013. Building the DLL fails because ActiveSocket.h is not found despite being present and the file being opened when clicking on it. I added the path for clsocket to the include path which allowed the build process to proceed. However I then get 37 errors, mostly (perhaps 90%) are these:

"definition of dllimport function not allowed"
"a function declared in dllimport may not be defined"

There are other errors and warnings. Compiling with Visual Studio 2017 requires that XP support be installed, but then fails in a similar manner. The output below is from Studio 2013:

Warning 1 warning C4005: 'IDI_SHIELD' : macro redefinition c:\temp\rsp_tcp\extio_rsp_tcp\src\resource.h 1487 1 ExtIO_RSP_TCP
Error 2 error C2491: 'InitHW' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 114 1 ExtIO_RSP_TCP
Error 3 error C2491: 'GetStatus' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 126 1 ExtIO_RSP_TCP
Error 4 error C2491: 'OpenHW' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 132 1 ExtIO_RSP_TCP
Error 5 error C2491: 'SetHWLO' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 153 1 ExtIO_RSP_TCP
Error 6 error C2491: 'StartHW' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 160 1 ExtIO_RSP_TCP
Error 7 error C2491: 'GetHWLO' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 181 1 ExtIO_RSP_TCP
Error 8 error C2491: 'GetHWSR' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 188 1 ExtIO_RSP_TCP
Error 9 error C2491: 'ExtIoGetSrates' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 194 1 ExtIO_RSP_TCP
Error 10 error C2491: 'ExtIoGetActualSrateIdx' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 206 1 ExtIO_RSP_TCP
Error 11 error C2491: 'ExtIoSetSrate' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 212 1 ExtIO_RSP_TCP
Error 12 error C2491: 'ExtIoGetSetting' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 230 1 ExtIO_RSP_TCP
Error 13 error C2491: 'ExtIoSetSetting' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 275 1 ExtIO_RSP_TCP
Error 14 error C2491: 'StopHW' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 335 1 ExtIO_RSP_TCP
Error 15 error C2491: 'CloseHW' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 351 1 ExtIO_RSP_TCP
Error 16 error C2491: 'ShowGUI' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 363 1 ExtIO_RSP_TCP
Error 17 error C2491: 'HideGUI' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 373 1 ExtIO_RSP_TCP
Error 18 error C2491: 'SwitchGUI' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 382 1 ExtIO_RSP_TCP
Error 19 error C2491: 'SetCallback' : definition of dllimport function not allowed C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 398 1 ExtIO_RSP_TCP
Warning 20 warning C4244: 'initializing' : conversion from 'LRESULT' to 'int', possible loss of data C:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 1084 1 ExtIO_RSP_TCP
Error 21 error C1017: invalid integer constant expression C:\Temp\RSP_TCP\ExtIO_RSP_TCP\clsocket\src\SimpleSocket.cpp ExtIO_RSP_TCP
22 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 113 31 ExtIO_RSP_TCP
23 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 125 29 ExtIO_RSP_TCP
24 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 131 31 ExtIO_RSP_TCP
25 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 152 30 ExtIO_RSP_TCP
26 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 159 29 ExtIO_RSP_TCP
27 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 180 30 ExtIO_RSP_TCP
28 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 187 30 ExtIO_RSP_TCP
29 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 193 29 ExtIO_RSP_TCP
30 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 205 30 ExtIO_RSP_TCP
31 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 211 30 ExtIO_RSP_TCP
32 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 229 31 ExtIO_RSP_TCP
33 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 274 31 ExtIO_RSP_TCP
34 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 334 30 ExtIO_RSP_TCP
35 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 350 30 ExtIO_RSP_TCP
36 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 362 30 ExtIO_RSP_TCP
37 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 372 31 ExtIO_RSP_TCP
38 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 381 31 ExtIO_RSP_TCP
39 IntelliSense: a function declared 'dllimport' may not be defined c:\Temp\RSP_TCP\ExtIO_RSP_TCP\src\ExtIO_RSP.cpp 397 30 ExtIO_RSP_TCP