Remove siteconf temp file once unused

Clean up the sitearch temp file before running any make commands to prevent
the temp file being copied into the gem installation.

Author: Dominic Cleal

lib/rubygems/ext/ext_conf_builder.rb

@@ -37,6 +37,7 @@ def self.build(extension, directory, dest_path, results, args=[])
         run cmd, results
 
         ENV["DESTDIR"] = nil
+        siteconf.unlink
 
         make dest_path, results
 

test/rubygems/test_gem_ext_ext_conf_builder.rb

@@ -34,6 +34,7 @@ def test_class_build
     assert_equal "creating Makefile\n", output[1]
     assert_contains_make_command '', output[2]
     assert_contains_make_command 'install', output[4]
+    assert_empty Dir.glob(File.join(@ext, 'siteconf*.rb'))
   end
 
   def test_class_build_rbconfig_make_prog