fix: preadv2 only available on GNU libc

Technically it is also available on musl libc 1.2.5,
yet the libc@0.2.153 (lastet) doesn't export the symbol,
and that version of musl libc is too new (released in 2024-03)

src/unix.rs

@@ -287,7 +287,10 @@ impl Client {
 
         // On Linux, we can use preadv2 to do non-blocking read,
         // even if `O_NONBLOCK` is not set.
-        #[cfg(target_os = "linux")]
+        //
+        // TODO: musl libc supports preadv2 since 1.2.5, but `libc` crate
+        // hasn't yet added it.
+        #[cfg(all(target_os = "linux", target_env = "gnu"))]
         {
             let read = self.read().as_raw_fd();
             loop {
@@ -577,7 +580,9 @@ extern "C" fn sigusr1_handler(
     // nothing to do
 }
 
-#[cfg(target_os = "linux")]
+// This should be available for all linux targets,
+// though only [`non_blocking_read`] currently uses it so adding gnu cfg.
+#[cfg(all(target_os = "linux", target_env = "gnu"))]
 fn cvt_ssize(t: libc::ssize_t) -> io::Result<libc::ssize_t> {
     if t == -1 {
         Err(io::Error::last_os_error())
@@ -586,7 +591,7 @@ fn cvt_ssize(t: libc::ssize_t) -> io::Result<libc::ssize_t> {
     }
 }
 
-#[cfg(target_os = "linux")]
+#[cfg(all(target_os = "linux", target_env = "gnu"))]
 fn non_blocking_read(fd: c_int, buf: &mut [u8]) -> io::Result<usize> {
     static IS_NONBLOCKING_READ_UNSUPPORTED: AtomicBool = AtomicBool::new(false);