Fix PKG_CONFIG environment variable setup
Move extra distributables into subdirectory
Introduce vbscript to check if/where HTML Help is installed

This allows htmlhelp to be installed anywhere on the system
and be shared by multiple build trees as well.

Note: it should *not* be installed anymore in HH_DIR !
Issue warning if html help workshop is installed on HH_DIR

And explain how to fix it for the future
Install HTML Help Workshop with elevated privileges starting from Windows Vista
Don't exit with error code - that will end install.sh