[ruby/fiddle] Don't use mkmf to generate dummy Makefile

(https://github.com/ruby/fiddle/pull/155)

GitHub: fix https://github.com/ruby/fiddle/pull/153

mkmf requires fileutils. JRuby doesn't like it.

https://github.com/ruby/fiddle/commit/df4bd21fc3

ext/fiddle/extconf.rb

@@ -1,11 +1,19 @@
 # frozen_string_literal: true
-require 'mkmf'
+
+require "rbconfig"
 
 unless RUBY_ENGINE == "ruby"
-  File.write('Makefile', dummy_makefile("").join)
+  File.write('Makefile', <<-MAKEFILE)
+all install clean:
+	#{RbConfig::CONFIG["NULLCMD"]}
+
+.PHONY: all install clean
+  MAKEFILE
   return
 end
 
+require 'mkmf'
+
 # :stopdoc:
 
 def gcc?