Undefined ruby_bcrypt() + util.h deprecation warning

I was not able to compile it successfully, due to an undefined reference to ruby_bcrypt() from https://github.com/codahale/bcrypt-ruby/blob/master/ext/mri/bcrypt_ext.c#L27.

Also, I was getting this: /usr/include/ruby-1.9.1/ruby/backward/util.h:2:2: warning: #warning use "ruby/util.h" instead of bare "util.h"