handle registry dependancies for html help executables to make cross platform

[barry-jones]

The HTML Help executables are used to compile the compiled html help files. These are only available on windows and when searching for the install directory the fallback search looks through the registry.

So as part of the move to the .NET core the compatabilty pack has been used. This will not work on non-windows platforms so needs some action to remove or not have these dependancies on other platforms.

[barry-jones]

Related to #3