Don't use mkmf to generate dummy Makefile

GitHub: fix GH-153

mkmf requires fileutils. JRuby doesn't like it.

Author: kou

ext/fiddle/extconf.rb

@@ -1,11 +1,17 @@
 # frozen_string_literal: true
-require 'mkmf'
 
 unless RUBY_ENGINE == "ruby"
-  File.write('Makefile', dummy_makefile("").join)
+  File.write('Makefile', <<-MAKEFILE)
+all install clean:
+	#{CONFIG["NULLCMD"]}
+
+.PHONY: all install clean
+  MAKEFILE
   return
 end
 
+require 'mkmf'
+
 # :stopdoc:
 
 def gcc?