build.rs: canonicalize CARGO_MANIFEST_DIR

We use this so you can run fish from the build directory and it picks
up its data files.

If this wasn't canonicalized, that would break if you're building with
a $PWD through a symlink.

build.rs

@@ -13,6 +13,16 @@ fn main() {
     let build_dir = format!("{}/build", env!("CARGO_MANIFEST_DIR"));
     let build_dir = option_env!("FISH_BUILD_DIR").unwrap_or(&build_dir);
     rsconf::set_env_value("FISH_BUILD_DIR", build_dir);
+    // We need to canonicalize (i.e. realpath)
+    // the manifest dir because we want to compare it simply as a string at runtime.
+    rsconf::set_env_value(
+        "CARGO_MANIFEST_DIR",
+        std::fs::canonicalize(env!("CARGO_MANIFEST_DIR"))
+            .unwrap()
+            .as_path()
+            .to_str()
+            .unwrap(),
+    );
 
     cc::Build::new()
         .file("src/libc.c")