Case-insensitive lookup of the PATH variable on Windows

Currently, the HEAD version is broken on Windows because it fails to look up the PATH environment variable. On all recent Windows versions that I checked (Win7/8) it's named Path, not PATH.

This change attempts to fix this issue on Windows.

As I'm absolute newbie in Cabal, so I don't think that this fix is the best one but I'm open to suggestions/ideas.

P.S. I'm a Haskell newbie as well, please be forgiving :)