Add build.rs to make CI happy

Author: Berrysoft

CHANGELOG.md

@@ -3,6 +3,9 @@
 - Add VxWorks support
   [#105](https://github.com/lambda-fairy/rust-errno/pull/105)
 
+- Add cygwin support
+  [#106](https://github.com/lambda-fairy/rust-errno/pull/106)
+
 # [0.3.10] - 2024-11-29
 
 - Update to windows-sys 0.59

build.rs

@@ -0,0 +1,4 @@
+fn main() {
+    // Cygwin target, added in 1.86
+    println!("cargo:rustc-check-cfg=cfg(target_os, values(\"cygwin\"))");
+}