Project spits an error, mentions wrong variable name to be set

```
    6>D:\prj\BazisLib\bzscore\bzscore.vcxproj(5,7): error : The WDK7_PATH environment variable is missing
       or does not point at Windows Driver Kit 7.1.0
```

The variable it requires to be set, however, is named `WDK7PATH` (no underscore!).
