change all WIN32 defines to _WIN32 as it is a compiler define
Merge pull request #63 from martell/master

Change all WIN32 defines to _WIN32.

"WIN32 is a name that one could use and even define in your own code and so might clash with Microsoft's usage. _WIN32 is a name that is reserved for the implementor (tn this case MS) because it begins with an underscore and an uppercase letter - you are not allowed to define reserved names in your own code, so there can be no clash."