cxx: fix build with CARGO_TARGET_DIR set

Author: fw-immunant

third_party/cxx/blobstore/build.rs

@@ -1,9 +1,16 @@
 fn main() {
+    let src_dir = {
+        let mut target_dir =
+            std::env::var_os("CARGO_TARGET_DIR").unwrap_or("../../../target".into());
+        target_dir.push("/cxxbridge/demo/src");
+        target_dir
+    };
+
     cxx_build::bridge("src/main.rs")
         .file("src/blobstore.cc")
         .flag_if_supported("-std=c++14")
         .include(".")
-        .include("../../../target/cxxbridge/demo/src")
+        .include(src_dir)
         .compile("cxxbridge-demo");
 
     println!("cargo:rerun-if-changed=src/main.rs");