Use Config::AutoConf to detect types

This is an updated (not broken) version of my previous PR. This changes the Makefile.PL to use Config::AutoConf to check for either "unsigned __int128" or  "unsigned int __attribute__ ((__mode__ (TI)))", rather than checking the compiler version.

I also did a bit of general cleanup in the Makefile.PL and added a .gitignore flie to the repo.