I'm seeing a Windows failure when trying to build current Dune master (with this PR). 

      (cd _build/default/otherlibs/xdg && C:\npm\prefix\node_modules\esy\node_modules\esy-bash\.cygwin\bin\x86_64-w64-mingw32-gcc.exe -O2 -fno-strict-aliasing -fwrapv -mms-bitfields -g -I C:/Users/runneradmin/.esy/3__________________________________________________________/i/ocaml-4.14.0-cd77a8d9/lib/ocaml -o xdg_stubs.o -c xdg_stubs.c)
  
      xdg_stubs.c: In function 'dune_xdg__get_known_folder_path':
  
      xdg_stubs.c:35:9: warning: implicit declaration of function 'SHGetKnownFolderPath'; did you mean 'SHGetFolderPath'? [-Wimplicit-function-declaration]
  
         res = SHGetKnownFolderPath(rfid, 0, NULL, &wcp);
  
               ^~~~~~~~~~~~~~~~~~~~
  
               SHGetFolderPath
  
      xdg_stubs.c:41:38: error: 'WC_ERR_INVALID_CHARS' undeclared (first use in this function); did you mean 'MB_ERR_INVALID_CHARS'?
  
         len = WideCharToMultiByte(CP_UTF8, WC_ERR_INVALID_CHARS, wcp, wlen, NULL, 0, NULL, NULL);
  
                                            ^~~~~~~~~~~~~~~~~~~~
  
                                            MB_ERR_INVALID_CHARS
  
      xdg_stubs.c:41:38: note: each undeclared identifier is reported only once for each function it appears in
  
      error: command failed: "dune" "build" "-p" "xdg" "-j" "4" "@install" (exited with 1)