mkconfig.rb: fix concatenated lines

* tool/mkconfig.rb: see if a variable is set before setting it to fix
  concatenated lines.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39129 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Makefile.sub: rubyarchdir in config.status

* win32/Makefile.sub (config.status): now rbconfig.rb needs
  rubyarchdir.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39130 b2dd03c8-39d4-4d8f-98ff-823fe69b080e