Make the mkmf methods private in the global [Bug #16896]

lib/mkmf.rb

@@ -2822,7 +2822,12 @@ realclean: distclean
   end
 end
 
-include MakeMakefile
+# MakeMakefile::Global = #
+m = Module.new {
+  include(MakeMakefile)
+  private(*MakeMakefile.public_instance_methods(false))
+}
+include m
 
 if not $extmk and /\A(extconf|makefile).rb\z/ =~ File.basename($0)
   END {mkmf_failed($0)}

test/mkmf/test_mkmf.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: false
+require 'test/unit'
+require 'mkmf'
+
+class TestMkmf < Test::Unit::TestCase
+  class TestGlobal < TestMkmf
+    main = TOPLEVEL_BINDING.receiver
+    MakeMakefile.public_instance_methods(false).each do |m|
+      define_method(:"test_global_#{m}") do
+        assert_respond_to(main, [m, true])
+        assert_not_respond_to(main, [m, false])
+      end
+    end
+  end
+end