Properly detect Linux with glibc to detect what's not

[illwieckz]

Properly detect Linux with glibc to detect what's not, no need to list everything else.

It makes it easier to build for something else we currently don't support, either being another operating system, or Linux with another libc.

I yet again faced the problem of assuming Glibc on Linux when toying with Fil-C that uses musl by default. I do remember having faced similar problems in the past with other stuff. Android? Zig? I don't remember precisely, but I knew that code was fragile, maybe it's better this way.

I'm curious to see if the CI will be happy or not.

If that works, then adding new operating systems would even require less code, not needing to list every of them.

[slipher]

• native_client: properly detect Linux with glibc to detect what's not LGTM
• FileSystem: properly detect Linux with glibc to detect what's not seems dubious. The *64 family of functions/structs is apparently defined in POSIX; I get hits for various commercial Unices, including MacOS since 10.5. So I don't see the motivation to associate it with glibc.
• FileSystem: let Native Client do open() like others hard to say since it appears to depend on the previous commit.