Fix $LOAD_PATH in rake and ext_conf builder

lib/rubygems/ext/builder.rb

@@ -54,6 +54,23 @@ def self.make(dest_path, results, make_dir = Dir.pwd, sitedir = nil, targets = [
     end
   end
 
+  def self.ruby
+    require "shellwords"
+    # Gem.ruby is quoted if it contains whitespace
+    cmd = Gem.ruby.shellsplit
+
+    # This load_path is only needed when running rubygems test without a proper installation.
+    # Prepending it in a normal installation will cause problem with order of $LOAD_PATH.
+    # Therefore only add load_path if it is not present in the default $LOAD_PATH.
+    load_path = File.expand_path("../..", __dir__)
+    case load_path
+    when RbConfig::CONFIG["sitelibdir"], RbConfig::CONFIG["vendorlibdir"], RbConfig::CONFIG["rubylibdir"]
+      cmd
+    else
+      cmd << "-I#{load_path}"
+    end
+  end
+
   def self.run(command, results, command_name = nil, dir = Dir.pwd, env = {})
     verbose = Gem.configuration.really_verbose
 

lib/rubygems/ext/ext_conf_builder.rb

@@ -22,8 +22,7 @@ def self.build(extension, dest_path, results, args=[], lib_dir=nil, extension_di
     destdir = ENV["DESTDIR"]
 
     begin
-      require "shellwords"
-      cmd = Gem.ruby.shellsplit << "-I" << File.expand_path("../..", __dir__) << File.basename(extension)
+      cmd = ruby << File.basename(extension)
       cmd.push(*args)
 
       run(cmd, results, class_name, extension_dir) do |s, r|

lib/rubygems/ext/rake_builder.rb

@@ -19,7 +19,7 @@ def self.build(extension, dest_path, results, args=[], lib_dir=nil, extension_di
       rake = rake.shellsplit
     else
       begin
-        rake = [Gem.ruby, "-I#{File.expand_path("../..", __dir__)}", "-rrubygems", Gem.bin_path("rake", "rake")]
+        rake = ruby << "-rrubygems" << Gem.bin_path("rake", "rake")
       rescue Gem::Exception
         rake = [Gem.default_exec_format % "rake"]
       end