Don't clean extension dir when installing gems.  Doing so breaks packaging
of some ports.

Index: lib/rubygems/ext/ext_conf_builder.rb
--- lib/rubygems/ext/ext_conf_builder.rb.orig
+++ lib/rubygems/ext/ext_conf_builder.rb
@@ -60,7 +60,6 @@ class Gem::Ext::ExtConfBuilder < Gem::Ext::Builder
         destent.exist? || FileUtils.mv(ent.path, destent.path)
       end
 
-      make dest_path, results, extension_dir, tmp_dest_relative, ["clean"], target_rbconfig: target_rbconfig
     ensure
       ENV["DESTDIR"] = destdir
     end
