Using a custom libc derivation

[theoparis]

Issue description

I have asked in several places but for some reason no one seems to have a proper solution to my problem. I want to use a custom libc with nixpkgs (for example, LLVM Libc, bionic or apple's libc built from upstream sources instead of prebuilt nixpkgs packages) and I cannot seem to find a way to override the libc easily by providing a derivation. All I could find is how to override it via a string such as libc = "musl" which doesn't help me here.

If this is not supported already, adding support for this could also make #296577 easier. Additionally, this could make porting libc++ or even nix to a new OS simpler.

If this is already supported I'd at least like for this be documented somewhere.

[RossComputerGuy]

Yeah, if being able to specify a custom libc was as simple as passing over a derivation then that could make the process just so much simpler.