Adding knowledge of libc-variant to alire

[AJ-Ianozi]

This is a continuation of my thoughts on https://github.com/alire-project/alire/issues/792#issuecomment-1758760974

Alire has knowledge of gettingos (linux, windows, macos, freebsd, etc), distribution (though I don't see alpine yet as an option), even host-arch (e.g. x86-64) but while Alire is able to be built with musl (such as on alpine) there's no easy way of detecting that from what I can see.

While most distros use glibc, it looks like quite a few are using musl now. I think it's worth adding a libc-variant (or even just libc) parameter (that defaults to being glibc?) or similar that's passed to alire or detected on build. Maybe then gprbuild's toml will look something like:

[origin."case(os)".linux."case(libc-variant)".glibc."case(host-arch)".x86-64]
url = "https://github.com/alire-project/GNAT-FSF-builds/releases/download/gprbuild-22.0.0-1/gprbuild-x86_64-linux-22.0.0-1.tar.gz"
hashes = ["sha256:24dfc1b54655edd4f85589e7e7dbd0bee24d087f25d5b0f13d3224fe7acf85b8"]

[origin."case(os)".linux."case(libc-variant)".musl."case(host-arch)".x86-64]
url = "https://github.com/alire-project/GNAT-FSF-builds/releases/download/gprbuild-22.0.0-1/gprbuild-x86_64-linux-alpine-22.0.0-1.tar.gz"
hashes = ["sha256:hashgoeshere"]

# Would this help for backwards-compatibility?  We default to glibc.

[origin."case(os)".linux."case(host-arch)".x86-64]
url = "https://github.com/alire-project/GNAT-FSF-builds/releases/download/gprbuild-22.0.0-1/gprbuild-x86_64-linux-22.0.0-1.tar.gz"
hashes = ["sha256:24dfc1b54655edd4f85589e7e7dbd0bee24d087f25d5b0f13d3224fe7acf85b8"]

What do you think? As a bonus, a libc-variant could someday be msvc or something on windows.