--- fish-4.3.1/build.rs.old	2025-12-29 19:08:59.244987404 -0500
+++ fish-4.3.1/build.rs	2025-12-29 19:09:23.384490623 -0500
@@ -76,6 +76,7 @@
         ("apple", &(|_| target_os_is_apple())),
         ("bsd", &(|_| target_os_is_bsd())),
         ("cygwin", &(|_| target_os_is_cygwin())),
+        ("illumos", &|_| env::var("CARGO_CFG_TARGET_OS").unwrap() == "illumos"),
         ("have_eventfd", &|target| {
             // FIXME: NetBSD 10 has eventfd, but the libc crate does not expose it.
             if target_os() == "netbsd" {