specify mingw g++ compiler for ruby windows artifact build

Rakefile

@@ -93,6 +93,7 @@ task 'dlls' do
 
   [ w64, w32 ].each do |opt|
     env_comp = "CC=#{opt[:cross]}-gcc "
+    env_comp += "CXX=#{opt[:cross]}-g++ "
     env_comp += "LD=#{opt[:cross]}-gcc "
     docker_for_windows "gem update --system && #{env} #{env_comp} make -j #{out} && #{opt[:cross]}-strip -x -S #{out} && cp #{out} #{opt[:out]}"
   end