By uncommenting the line changed by this commit, `ruby -c build_config/default.rb` complains of a syntax error due to the illegally nested double quotes
If we modify an option that may have been specified by users, we may
make unintended changes, so it is better not to modify it as much as
possible, IMO.
In addition, update the documents referring `build_config.rb` which is
no longer used. The new `build_config.rb` describes the new configuration
structure in the comment.