ext_conf_builder.rb: use intermediate destdir always

* lib/rubygems/ext/ext_conf_builder.rb (Gem::Ext::ExtConfBuilder.build):
  it is impossible to predict which file will be installed to where,
  by the arguments, so use intermediate destination directory always.
  [Bug #7698]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39696 b2dd03c8-39d4-4d8f-98ff-823fe69b080e