Windows fixes.  There were some strange things going on with Windows
installation directories - I'm sure I've mucked up whatever was
happening with 'make install' for GHC on Windows, but there were
defintitely some bugs in here.  eg. we shouldn't have been setting
datadir to $(prefix)/imports unconditionally on Windows.

Make install now works for Happy, Alex and Haddock.  GHC probably
needs a bit of tweaking to get it working again.