Dinking around with building windows DLLs with MinGW again. And figured out where the [[gnu::stdcall]] goes.

build.bat

g++ -std=c++11 -Wall -Wextra -c dlltest.cpp
g++ -shared -static-libgcc -static-libstdc++ -o dlltest.dll dlltest.o -Wl,--add-stdcall-alias

dlltest.cpp

extern "C" {
    [[gnu::dllexport]] int add [[gnu::stdcall]] (int x, int y) noexcept
    {
        return x + y;
    }
}