Support environment variables

The way we work around SR-1636 is we add a C language target with atbuild's new C project syntax.

This C target can access `environ` and the result follows.

I think this bootstraps OK, we'll see what CI says

See also: https://bugs.swift.org/browse/SR-1636, https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160523/019350.html
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