Few improvements

`add_include_path` and `add_define` methods so that when compiling can specify those in Rakefile


Add load path where `rb_file` is located so that `require` works correctly for export.


Remove `/dev/null` as it wont work in Windows, it will complain that file not found. In Windows it's `> nul` but that won't work for Linux, so we just save output to file and because that directory is tmp it will be deleted after anyway.