Unbundle strndup() on all Unix-like systems

--- ../libssh-rs-7d884fa4e506b7273304e6c4452f928ca7a28198/libssh-rs-sys/build.rs.orig	2021-12-16 04:55:11 UTC
+++ ../libssh-rs-7d884fa4e506b7273304e6c4452f928ca7a28198/libssh-rs-sys/build.rs
@@ -87,13 +87,11 @@ fn main() {
         cfg.define("HAVE_TERMIOS_H", Some("1"));
         cfg.define("HAVE_UNISTD_H", Some("1"));
         cfg.define("HAVE_VSNPRINTF", Some("1"));
+        cfg.define("HAVE_STRNDUP", Some("1"));
 
         if !target.contains("darwin") {
             cfg.define("HAVE_POLL", Some("1"));
         }
-    }
-    if target.contains("linux") {
-        cfg.define("HAVE_STRNDUP", Some("1"));
     }
     if target.contains("darwin") {
         cfg.define("HAVE_NTOHLL", Some("1"));