Make the NSIS output automatically select the output architecture to
generate the proper filename and, much more importantly, set up the
proper default install directory.
This requires Perl as well as makensis to be present in order to make
an installer, but that doesn't really seem like a too onerous of a
requirement (NSIS being the big external dependency here.)
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
