Support compiling against Musl

[simonjbeaumont]

Motivation

When building for Linux, we would like to be able to build this package on top of Musl, as well as the usual Glibc.

Modifications

Replace imports of Glibc with conditional imports using the following pattern:

- import Glibc
+ #if canImport(Glibc)
+ import Glibc
+ #elseif canImport(Musl)
+ import Musl
+ #else
+ #error("Unsupported runtime")
+ #endif

Result

Can compile against Musl.

[FranzBusch]

@swift-server-bot test this please

[FranzBusch]

I am happy to get this in but we should see that projects start to drop this dependency since it is no longer needed.

[simonjbeaumont]

@FranzBusch Agreed. But it would still be useful in the short term as it is used in a few places where I'm sprinkling this change around.

[simonjbeaumont]

Actually, I think we don't need this one after all :)