Cleanup mingw build

Attached you can find some cleanup regarding the mingw build process.

The cross build can still be done per ```rake gem:windows``` or by the [build_all script](https://github.com/sparklemotion/nokogiri/blob/master/build_all) within the rake-compiler-dev-box . It can now be done per standard ```rake cross native gem``` command, too. It can not yet be done per ```package_win32_fat_binary``` command, but I created an issue on this: https://github.com/tjschuck/rake-compiler-dev-box/issues/8 .