Variants for Windows with working run-scripts

[zah]

Most of this code was originally authored by
Robert Pettersen [email protected] (@njubee at github)

I rebased it on the latest tup revision, removed some of the
superficial changes and incorporated the unicode-related fixed
from the tup repository.

[trap15]

This is a big blocker for various projects of mine. I've been using the unofficial win32 build of this branch for years now which is great, except now that I have to use GCC from a tupfile it's no longer working, due to mixing forward and backslashes. Of course the simple solution would just be to pull this PR locally and re-compile tup to only use forward slashes, but perhaps if I show my support for this it could possibly get merged and I'd feel less bad about it.

Basically what I'm saying is I would greatly appreciate this PR getting reviewed at some point. It's a shame Tup seems like abandonware at this point because it's by far the best build system I've ever used, and I'd like to keep using it.

[trap15]

I don't like to double-post, but seeing as there appears to be traction gaining for tup on windows... can I beg for this to get updated and merged? :)

[HaoZeke]

I hope this gets merged soon..