Let me chime in, before you get yourselves in another mess
For your information: As there were no more comments on this, I started working on a tool now, which transforms .spec files into both GNU and MSVC .def files.
You _do_ know that Microsoft's linker tools have only limited support for .def files, right? That you cannot create __stdcall exports without also exporting the decoration, unless the functions were marked as __declspec(dllexport)? and that in the latter case it's much easier to let the linker process the compiled .obj files to extract the dllexport directives to produce a .exp and a .lib file? hence, that the best way to produce import/export libraries is to turn the .spec file not into a .def, but into a .c/.cpp with dummy implementations of each function and a __declspec(dllexport) attribute, compile it and run the linker on the generated .obj file? Spare yourself the pain I went through. Think about it