feat: verify FALLBACK_INCLUDE_PATH in build.rs

Author: komar007

build.rs

@@ -0,0 +1,11 @@
+use std::path::Path;
+
+fn main() {
+    if let Some(path) = option_env!("FALLBACK_INCLUDE_PATH") {
+        let path = Path::new(path);
+        assert!(
+            path.is_absolute(),
+            "Environment variable FALLBACK_INCLUDE_PATH must be absolute: {path:?}"
+        );
+    }
+}