mkmf.rb: expand RUBY_SO_NAME

* lib/mkmf.rb (RbConfig): expand RUBY_SO_NAME for extensions
  backward compatibility.  [ruby-core:59426] [Bug #9329]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44469 b2dd03c8-39d4-4d8f-98ff-823fe69b080e