* lib/rubygems/ext/ext_conf_builder.rb (Gem::Ext::ExtConfBuilder.build): clear DESTDIR so RUBYARCHDIR and RUBYLIBDIR are not be overrdden. [Bug #7698] git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39592 b2dd03c8-39d4-4d8f-98ff-823fe69b080e