packaging: Fix installation scripts on windows/RelocatableBuild case

This includes quite a lot of small fixes which fix the installation
makefile to work on windows properly. This also required fixing the
RelocatableBuild variable which seemed to have been broken for a long
while.

Sam helped me a lot writing this patch by providing a windows machine to
test the changes. Without him it would have taken ages to tweak
everything.

Co-authored-by: sheaf <sam.derbyshire@gmail.com>