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.

[illwieckz]

I merged the first commit.

[illwieckz]

• 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.

Possibly, but that doesn't prevent us to cleanup the code. We were already avoiding the *64 family outside of Linux, so that code changes nothing for other unices like macOS, etc. What this patch does is to avoid the *64 family on Linux when glibc isn't there.

In the future we can add more tests to enable *64 functions on more systems.

[illwieckz]

I tested macOS and open64 and others don't exist. Actually we may avoid open64() on Linux.

[illwieckz]

It looks like we don't need open64(), as glibc remaps open() as open64() and other symbols when _FILE_OFFSET_BITS 64 is set. Other libc (like musl) may only provide a 64-bit large file implementation anyway.

[illwieckz]

The macOS manpage mentions things like fstat64() but as deprecated.

[slipher]

Oops, found a pending comment from weeks ago.

[slipher]

I would expect this not to work correctly since when precompiled headers are used, the PCH is inserted before the file contents. Anyway that sort of thing would be better added from the build system.

Also please squash file offset size-related commits together.