Silence unexpected_cfgs error about unknown cygwin target

Extract our own cfg value, to avoid noisy warnings like:

	warning: unexpected `cfg` condition value: `cygwin`
	  --> src/fallback.rs:78:23
	   |
	78 |             #[cfg(not(target_os = "cygwin"))]
	   |                       ^^^^^^^^^^^^^^^^^^^^
	   |

The cygwin target will be added to Rust 1.86, so we can get rid of this
after some time.

build.rs

@@ -90,6 +90,7 @@ fn detect_cfgs(target: &mut Target) {
         ),
         ("apple", &detect_apple),
         ("bsd", &detect_bsd),
+        ("cygwin", &detect_cygwin),
         ("gettext", &have_gettext),
         ("small_main_stack", &has_small_stack),
         // See if libc supports the thread-safe localeconv_l(3) alternative to localeconv(3).
@@ -128,6 +129,11 @@ fn detect_apple(_: &Target) -> Result<bool, Box<dyn Error>> {
     Ok(cfg!(any(target_os = "ios", target_os = "macos")))
 }
 
+#[allow(unexpected_cfgs)]
+fn detect_cygwin(_: &Target) -> Result<bool, Box<dyn Error>> {
+    Ok(cfg!(target_os = "cygwin"))
+}
+
 /// Detect if we're being compiled for a BSD-derived OS, allowing targeting code conditionally with
 /// `#[cfg(bsd)]`.
 ///