mkmf.rb: RUBY_VERSION_NAME

* lib/mkmf.rb (MakeMakefile#configuration): set all ruby names.
  hdrdir now needs RUBY_VERSION_NAME.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39147 b2dd03c8-39d4-4d8f-98ff-823fe69b080e