Comment out Math module and Time class for user friendly

build_config.rb is generated by mgem as much as possible. If "Math module" and "Time class" were written in default build_config.rb, mgem will delete these config lines in ordinary below usage.

mgem config > build_config.rb

I'ts not user-friendly.
so, I think we shuould prepare minimal build_config.rb, and add setting of recommended standard module or class to mgem interactive settings like ToolChain. Cc: @bovi

How do you think?