Add RHEL 5 compat

src/util_libc.rs

@@ -128,10 +128,13 @@ cfg_if! {
 // SAFETY: path must be null terminated, FD must be manually closed.
 pub unsafe fn open_readonly(path: &str) -> Option<libc::c_int> {
     debug_assert!(path.as_bytes().last() == Some(&0));
-    // We don't care about Linux OSes too old to support O_CLOEXEC.
     let fd = open(path.as_ptr() as *mut _, libc::O_RDONLY | libc::O_CLOEXEC);
     if fd < 0 {
         return None;
     }
+    // O_CLOEXEC works on all Unix targets except for older Linux kernels (pre
+    // 2.6.23), so we also use an ioctl to make sure FD_CLOEXEC is set.
+    #[cfg(target_os = "linux")]
+    libc::ioctl(self.fd, libc::FIOCLEX);
     Some(fd)
 }