Use specific version of libc

[Susurrus]

It should be policy for additional functionality to wait for a libc release anyways before nix can release, so these should just bump the libc dependency version anyways.

[Susurrus]

Probably #565 should be merged instead of this, though a policy should be set on whether we want to depend only on released versions for dependencies or using git directly is fine. @posborne @fiveop Any comments on this or should I bring it up on nix-rust/rfcs?

[kamalmarhubi]

Right now we set a fixed version at release time, and otherwise leave it tracking the repository. This allows someone to add a feature to nix and libc without requiring a release of libc until we want to release nix. It's weird, but I think it's a nice way not to force lots of libc releases to make progress on nix.

[kamalmarhubi]

I merged #565 so I'm going to close this. But please reopen and/or start a new discussion if you want to chat more about it!