You should really be switching to MSYS2/mingw-w64-i686 for Windows build.
The only optional lib that is missing from packaging ecosystem is SDL_sound. Everything else should have already been available. You don't need to compile libpng, SDL, and SDL_net from source.
If you really want SDL_sound with DOSBox SVN, then compiling SDL_sound on MSYS2/mingw-w64-i686 is also a piece of cake, because all the SDL_sound dependecies libs (for a full-featured SDL_sound) are also available from packaging ecosystem. You also get the latest GCC 7.3.0 from MSYS2/mingw-w64-i686.
MinGW.org does not seem to be maintained and updated anymore. Last GCC version is 6.3.0, and there are chains of dependencies libs to resolve for building DOSBox SVN. Although it is still doable provided one is adventurous enough to go through the process, I would suggest that you spare yourself the trouble and just use MSYS2/mingw-w64-i686.