This diff adds NSIS to the windows package generators, and configure it
to create a windows graphical installer/uninstaller.
The generated installer is very similar to the one from autotools, the
main difference is that this installer asks for licence agreement.
The installed files differ from autotools, which installs a lot of files
in a custom subtree, including documentation on how to build the
software and a lot of things which are unexpected output from a
graphical installer. This one only installs the files from the package,
identical to what is distributed as part of a release.
Depends on D4284.