added support for GNU/Hurd

Author: xevor11

build.rs

@@ -33,7 +33,7 @@ const ALLOWED_CFGS: &'static [&'static str] = &[
 
 // Extra values to allow for check-cfg.
 const CHECK_CFG_EXTRA: &'static [(&'static str, &'static [&'static str])] = &[
-    ("target_os", &["switch", "aix", "ohos"]),
+    ("target_os", &["switch", "aix", "ohos", "hurd"]),
     ("target_env", &["illumos", "wasi", "aix", "ohos"]),
     (
         "target_arch",

src/unix/hurd/align.rs

@@ -0,0 +1 @@
+// Placeholder file