Build using relative path in repo if Windows lost symlink

serde_derive_internals/build.rs

@@ -0,0 +1,12 @@
+use std::path::Path;
+
+fn main() {
+    // Sometimes on Windows the git checkout does not correctly wire up the
+    // symlink from serde_derive_internals/src to serde_derive/src/internals.
+    // When this happens we'll just build based on relative paths within the git
+    // repo.
+    let mod_behind_symlink = Path::new("src/mod.rs");
+    if !mod_behind_symlink.exists() {
+        println!("cargo:rustc-cfg=serde_build_from_git");
+    }
+}