ext_conf_builder.rb: clear DESTDIR
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39592 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
ext_conf_builder.rb: clear DESTDIR
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