http://www.rubyinside.com/how-to-create-a-ruby-extension-in-c-in-under-5-minutes-100.html

.gitignore

Makefile
*bundle
*o

extconf.rb

require 'mkmf'

extension_name = 'MacRuby'

dir_config(extension_name)
create_makefile(extension_name)

MacRuby.c

#include "ruby.h"

VALUE MacRuby = Qnil;

void Init_MacRuby();

VALUE version(VALUE self);

void Init_MacRuby() {
  MacRuby = rb_define_class("MacRuby", rb_cObject);
  rb_define_singleton_method(MacRuby, "version", version, 0);
}

VALUE version(VALUE self) {
  return INT2NUM(1);
}