Add unpacktool to CMake, port to *nix

- Adds missing includes, removes some unused ones.
- Removes unused Windows.h and shellapi.h includes.
- Uses / as path separator. This is portable and should still work fine
  on Windows.

unpacktool/PrefixCode.cpp

@@ -1,3 +1,5 @@
+#include <climits>
+
 #include "PrefixCode.h"
 
 struct XADCodeTreeNode