Lesson for the day, when installing Ruby DevKit on Windows ensure that the Ruby installation architecture matches the DevKit architecture.
ruby --version
ruby 2.0.0p247 (2013-06-27) [x64-mingw32]
Note the [x64-mingw32], you will get some strange compilation errors trying to install gems using the 32 bit DevKit with 64 bit Ruby (and vice-versa).
