ext_conf_builder.rb: install via temporary directory

* lib/rubygems/ext/ext_conf_builder.rb (Gem::Ext::ExtConfBuilder.build):
  fix for unusal cases again.  install to a temporary directory once
  and move instaled files to the destination directory, if it is same
  as the current directory.  [Bug #7698]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39579 b2dd03c8-39d4-4d8f-98ff-823fe69b080e