mingw-w64 generate and use a dll .def file and implib

download the sources and build expdef.exe

  i686-w64-mingw32-g++-win32 -static -m32 -Wall -O2 -o expdef.exe expdef.cpp

generate the .def and the libsomething.dll.a implib

  wine ./expdef.exe -p -o something.dll >something.def
  i686-w64-mingw32-dlltool --no-leading-underscore -d something.def -D something.dll -l libsomething.dll.a

if something.dll use stdcall convention the sources need to prefix the prototype declarations with [[gnu::stdcal]]

  [[gnu::stdcall]] void *SomeFunctionInSomething(void);

build and link the executable

  i686-w64-mingw32-g++-win32 -Wall -O2 -I./include -o someprogram.exe someprogram.cpp -L. -lsomething

That’s it.