build.rs: Use rsconf::declare_dependency()

This new feature in rsconf 0.2.0 resolves the compile-time warnings we get under
rustc 1.80+ about unrecognized cfg names by informing cargo of all valid cfg
names/values even when the cfg in question isn't enabled.

build.rs

@@ -92,8 +92,7 @@ fn detect_cfgs(target: &mut Target) {
     ] {
         match handler(target) {
             Err(e) => rsconf::warn!("{}: {}", name, e),
-            Ok(true) => rsconf::enable_cfg(name),
+            Ok(enabled) => rsconf::declare_cfg(name, enabled),
-            Ok(false) => (),
         }
     }
 }