added cross compiling with Ruby 2.0 (tested against mingw-w64 4.6.3 on Ubuntu 12.04)

This is to enable the use of Ruby 2.0 on Windows without having to vendor the mingw binary.

I have also added the patches needed for cross-compiling 1.8.6 and 1.9.1 against the same mingw version. Refer to [this gist](https://gist.github.com/prusswan/8246120) for the list of official backports contained in the patches.