Function libc 

pub fn libc(target: TargetTuple) -> Libc

Which of the four cases this target is.

Asked in two places, which is why it is a function rather than a match in each: LinkLine uses it to pick the files and crate::argv::argv uses it to refuse a static link against a stub. Two copies of this rule would be two rules.

The format is asked before the environment, because what holds a libc’s names is a property of the object format and Env::Gnu means mingw-w64 on a Windows target and glibc on a Linux one.