install.c: support for a 'pre-install-script', run before anything has
been installed. Provides a 'message_box' module function for use by
either the pre-install or post-install scripts.

bdist_wininst.py: support for pre-install script. Typo (build->built),
fixes so that --target-version can still work, even when the
distribution has extension modules - in this case, we insist on
--skip-build, as we still can't actually build other versions.