Fixed installing gems with extensions under -V

Commit f815636 changed the return value of ExtConfBuilder#build to nil
from the input result array.  When running with -V RubyGems prints the
results of compilation by joining the returned Array.  nil does not have
this so a NoMethodError was raised.

Fixes #601

lib/rubygems/ext/ext_conf_builder.rb

@@ -48,14 +48,14 @@ def self.build(extension, directory, dest_path, results)
             destent.exist? or File.rename(ent.path, destent.path)
           end
         end
-
-        results
       ensure
         ENV["RUBYOPT"] = rubyopt
         ENV["DESTDIR"] = destdir
       end
     end
     t.unlink if t and t.path
+
+    results
   ensure
     FileUtils.rm_rf tmp_dest if tmp_dest
   end

test/rubygems/test_gem_ext_ext_conf_builder.rb

@@ -27,7 +27,10 @@ def test_class_build
     output = []
 
     Dir.chdir @ext do
-      Gem::Ext::ExtConfBuilder.build 'extconf.rb', nil, @dest_path, output
+      result =
+        Gem::Ext::ExtConfBuilder.build 'extconf.rb', nil, @dest_path, output
+
+      assert_same result, output
     end
 
     assert_match(/^#{Gem.ruby} extconf.rb/, output[0])