Fix for installation of HTML Help Workshop on Windows Vista & more recent

[Geert Janssens]

I have just pushed a number of commits to gnucash-on-windows that hopefully fixes the issue 
that HTML Help Workshop could not be installed on Windows Vista and more recent due to 
insufficient privileges.

I don't have such a Windows system so I need some volunteers to test it and give feedback.

The way to test would be to set up a development environment on Windows following the 
instructions here:
https://github.com/Gnucash/gnucash-on-windows/blob/master/README

If you already have a development environment set up based on the gnucash-on-windows 
repository you can skip the basic set up and just remove HTML Help Workshop via the Windows 
control panel and rerun install.sh

What I hope is that you can successfully install HTML Help Workshop when during the 
corresponding step. On Windows Vista and more recent I expect Windows to show a UAC which 
you should grant.

In all cases, please keep in mind that you should *NOT* install HTML Help Workshop in the 
gcdev directories. Just install it in the default location somewhere under Program Files or similar. 
This has the added benefit that one single installation is ever needed. If you set up multiple 
development environments for gnucash, they will all use the same HH installation.

I'm looking forward for all feedback.

Regards,

Geert