Fix errno location on DragonFly BSD

DragonFlyBSD uses a thread-local variable for errno. libstd uses a
feature (`thread_local`) which is not stablized [1].

[1] rust-lang/rust#29594

serial-unix/Cargo.toml

@@ -14,3 +14,6 @@ categories = ["hardware-support", "os", "os::unix-apis"]
 [dependencies]
 serial-core = { version = "0.4", path = "../serial-core" }
 libc = "0.2.33"
+
+[target.'cfg(target_os="dragonfly")'.dependencies]
+errno-dragonfly = "0.1.1"

serial-unix/src/error.rs

@@ -28,6 +28,9 @@ use std::str;
 
 use libc::{c_int, c_char};
 
+#[cfg(target_os = "dragonfly")]
+use dfly::errno_location;
+
 pub fn last_os_error() -> core::Error {
     from_raw_os_error(errno())
 }
@@ -73,23 +76,12 @@ extern {
     fn errno_location() -> *mut c_int;
 }
 
-#[cfg(not(target_os = "dragonfly"))]
 pub fn errno() -> i32 {
     unsafe {
         (*errno_location()) as i32
     }
 }
 
-#[cfg(target_os = "dragonfly")]
-pub fn errno() -> i32 {
-    extern {
-        #[thread_local]
-        static errno: c_int;
-    }
-
-    unsafe { errno as i32 }
-}
-
 pub fn error_string(errno: i32) -> String {
     extern {
         #[cfg_attr(any(target_os = "linux", target_env = "newlib"),

serial-unix/src/lib.rs

@@ -24,6 +24,9 @@
 extern crate serial_core as core;
 extern crate libc;
 
+#[cfg(target_os = "dragonfly")]
+extern crate errno_dragonfly as dfly;
+
 pub use tty::*;
 
 mod error;