Hacked 'set_final_options()' to set (hopefully) appropriate values for

'install_site_lib' and install_site_platlib' on non-POSIX platforms.
Should at least work for NT, as this is adopted from Amos Latteier's NT
patches.  Also added extensive comments bitching about the inadequacy of
the current model, both under POSIX and NT (and probably other) systems.