Provide a way to tweak libstd compilation dependency resolution

[RalfJung]

Right now, compiling libstd of the latest nightly with xargo is broken. I submitted rust-lang/rust#43975 to rust to fix this, but it would be nice to have a fix (if this comes up again) that doesn't take many days to land.

The problem is that libstd depends on gcc = "0.3.50". rust's Cargo.lock makes sure that version 0.3.50 is used. However, with xargo, the lock file isn't available and it will happily use newer versions from crates.io if they are semver-compatible. Currently, it picks 0.3.52. The problem is that this version adds deprecations, and part of libstd have deny(warnings), so using deprecated items makes the build fail.

If I had some more control over the Cargo.toml that xargo generates to build libstd, I would work around this by adding the following to that file:

[dependencies.gcc]
version = "=0.3.50"

I thought this should also be possible with the [patch] or [replace] features, but I could not get them to work here, so maybe I am mistaken.

Do you think it makes sense for xargo to support something like this?

[RalfJung]

Closing in favor of #162, which is the better way to fix this problem.