Make Platform detection work with cross compilers

Rust/MelonProxy/build.rs

@@ -1,5 +1,20 @@
-#[cfg(windows)]
+use std::env;
+
 fn main() {
+    let target_os = env::var("CARGO_CFG_TARGET_OS");
+
+    match target_os.as_ref().map(|x| &**x) {
+        Ok("linux") | Ok("android") => {},
+        Ok("freebsd") | Ok("dragonfly") => {},
+        Ok("openbsd") | Ok("bitrig") | Ok("netbsd") | Ok("macos") | Ok("ios") => {}
+
+        Ok("windows") => link_exports(),
+
+        tos => panic!("unknown target os {:?}!", tos)
+    }
+}
+
+fn link_exports() {
     println!("cargo:warning=Linking Exports File..");
     use std::path::Path;
     let lib_path = Path::new("deps").join("Exports.def");
@@ -8,9 +23,4 @@ fn main() {
         "cargo:rustc-cdylib-link-arg=/DEF:{}",
         absolute_path.display()
     );
-}
-
-#[cfg(not(windows))]
-fn main() {
-
 }