optimizer: add missing "else"

Thanks to David Vanderson
ignore .sl.cache files

Created by `msbuild`?
win32: support symbolic links

Windows supports symbolic links in Vista and later.
win64: fix fixnum-to-extfl conversion
windows: fix symbolic link handling to match the OS

Windows parses relative-path links with yet another set of rules ---
slightly different from the many other existing rules for parsing
paths. Unfortunately, a few OS calls don't provide an option for
having the OS follow links, so we have to re-implement (our best guess
at) the OS's parsing of links.
distro-build: allow any string for `#:vc`
unbreak build

I have no idea whether the change is generally right, but it allows
the build to complete and DrRacket to start.