Look up the language module

Look up language module with `MakeMakefile.[]`, insted of a
accessing constant under that module directly, to get rid of
expose the constant to the toplevel inadvertently.

ext/-test-/cxxanyargs/extconf.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: false
 
-cxx = MakeMakefile::CXX
+cxx = MakeMakefile["C++"]
 
 ok = cxx.try_compile(<<~'begin', "") do |x|
   #include "ruby/config.h"

lib/mkmf.rb

@@ -2776,7 +2776,17 @@ distclean: clean distclean-so distclean-static distclean-rb-default distclean-rb
 realclean: distclean
 "
 
-  module CXX
+  @lang = Hash.new(self)
+
+  def self.[](name)
+    @lang.fetch(name)
+  end
+
+  def self.[]=(name, mod)
+    @lang[name] = mod
+  end
+
+  self["C++"] = Module.new do
     include MakeMakefile
     extend self