bug#25327: cargo build-system should be able to filter out target.cfg(windows) dependencies

[ng0]

The cargo build-system should be able to filter out Windows
dependencies.
Ignore blocks with this in Cargo.toml files:
[target.'cfg(windows)'.dependencies]

until the next [ ] starts.

I also found that most *32-sys packages (currently ALL *32-sys
I've seen) are windows specific.
As the build system is simple enough to not complain, you just
have to learn about this while doing the work.

-- 
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