ext_conf_builder.rb: remove circular dependency in install-so

* lib/rubygems/ext/ext_conf_builder.rb
  (Gem::Ext::ExtConfBuilder.hack_for_obsolete_style_gems): remove
  circular dependencies in install-so too.  [ruby-core:52882]
  [Bug #7698]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39572 b2dd03c8-39d4-4d8f-98ff-823fe69b080e