24e5fd3bed fs: remove _POSIX_C_SOURCE defining (fanquake)
Pull request description:
On Linux systems, `_POSIX_C_SOURCE` will default to `200809L` (since glibc 2.10). There's currently no reason for us to undefine it, and then set it to an earlier value. Also tested with musl libc.
I think if anything, the project should be settings macros like `_POSIX_C_SOURCE`, globally.
ACKs for top commit:
hebasto:
re-ACK 24e5fd3bed, only rebased since my recent [review](https://github.com/bitcoin/bitcoin/pull/32460#pullrequestreview-2854183748).
Tree-SHA512: 920d60058821992193616e0c73c2f7e4230a9e3ccb9d71d16493ae69696c868f4325d3dd2d4e8388749080c187aa7b205493b3e2c6986ad37440e591ebe107e1
On Linux systems, `_POSIX_C_SOURCE` will default to `200809L` (since
glibc 2.10). There's currently no reason for us to undefine it, and then
set it to an earlier value. Also tested with musl libc.
I think if anything, the project should be settings macros like
`_POSIX_C_SOURCE`, globally.
It's probably debatable if we support targets like iOS, but for now,
document why we are checking for this standard library feature.
Trying to use `std::system` for a `aarch64-darwin-ios` target results in
```bash
test.cpp:7:10: error: 'system' is unavailable: not available on iOS
7 | std::system("some_command");
| ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/_stdlib.h:203:6: note: 'system' has been explicitly marked unavailable here
203 | int system(const char *) __DARWIN_ALIAS_C(system);
| ^
1 error generated.
```
We really just want to skip this when building for Windows. So do that,
and remove the two header checks (we also already use both of these
headers, unguarded, in the !windows part of the codebase).
Squash the two *iffaddrs defines into one, as I haven't seen an
iffaddrs.h that implements one, but not the other.
