[rubygems/rubygems] Don't modify RbConfig at all when building extensions

Instead, pass sitearchdir and sitelibdir directly to `make`.

This also removes the need to create and use the siteconf file at all
when generating makefiles.

https://github.com/rubygems/rubygems/commit/dea41fa2dc

lib/rubygems/ext/builder.rb

@@ -17,7 +17,7 @@ class Gem::Ext::Builder
     $1.downcase
   end
 
-  def self.make(dest_path, results, make_dir = Dir.pwd)
+  def self.make(dest_path, results, make_dir = Dir.pwd, sitedir = nil)
     unless File.exist? File.join(make_dir, 'Makefile')
       raise Gem::InstallError, 'Makefile not found'
     end
@@ -33,11 +33,18 @@ class Gem::Ext::Builder
     # The installation of the bundled gems is failed when DESTDIR is empty in mswin platform.
     destdir = (/\bnmake/i !~ make_program_name || ENV['DESTDIR'] && ENV['DESTDIR'] != "") ? 'DESTDIR=%s' % ENV['DESTDIR'] : ''
 
+    env = [destdir]
+
+    if sitedir
+      env << 'sitearchdir=%s' % sitedir
+      env << 'sitelibdir=%s' % sitedir
+    end
+
     ['clean', '', 'install'].each do |target|
       # Pass DESTDIR via command line to override what's in MAKEFLAGS
       cmd = [
         *make_program,
-        destdir,
+        *env,
         target,
       ].reject(&:empty?)
       begin

lib/rubygems/ext/ext_conf_builder.rb

@@ -23,26 +23,13 @@ class Gem::Ext::ExtConfBuilder < Gem::Ext::Builder
     # Details: https://github.com/rubygems/rubygems/issues/977#issuecomment-171544940
     tmp_dest_relative = get_relative_path(tmp_dest.clone, extension_dir)
 
-    Tempfile.open %w[siteconf .rb], extension_dir do |siteconf|
-      siteconf.puts "require 'rbconfig'"
-      siteconf.puts "dest_path = #{tmp_dest_relative.dump}"
-      %w[sitearchdir sitelibdir].each do |dir|
-        siteconf.puts "RbConfig::MAKEFILE_CONFIG['#{dir}'] = dest_path"
-      end
-
-      siteconf.close
-
     destdir = ENV["DESTDIR"]
 
     begin
-        # workaround for https://github.com/oracle/truffleruby/issues/2115
-        siteconf_path = RUBY_ENGINE == "truffleruby" ? siteconf.path.dup : siteconf.path
       require "shellwords"
-        cmd = Gem.ruby.shellsplit << "-I" << File.expand_path('../..', __dir__) <<
+      cmd = Gem.ruby.shellsplit << "-I" << File.expand_path('../..', __dir__) << File.basename(extension)
-              "-r" << get_relative_path(siteconf_path, extension_dir) << File.basename(extension)
       cmd.push(*args)
 
-        begin
       run(cmd, results, class_name, extension_dir) do |s, r|
         mkmf_log = File.join(extension_dir, 'mkmf.log')
         if File.exist? mkmf_log
@@ -54,12 +41,10 @@ class Gem::Ext::ExtConfBuilder < Gem::Ext::Builder
           FileUtils.mv mkmf_log, dest_path
         end
       end
-          siteconf.unlink
-        end
 
       ENV["DESTDIR"] = nil
 
-        make dest_path, results, extension_dir
+      make dest_path, results, extension_dir, tmp_dest_relative
 
       full_tmp_dest = File.join(extension_dir, tmp_dest_relative)
 
@@ -77,8 +62,6 @@ class Gem::Ext::ExtConfBuilder < Gem::Ext::Builder
       end
     ensure
       ENV["DESTDIR"] = destdir
-        siteconf.close!
-      end
     end
 
     results