Add cpp flag to cc

This means that clang++ will be called instead of clang.
diff --git a/build.rs b/build.rs
index 1f3b6eb..05d7880 100644
--- a/build.rs
+++ b/build.rs
@@ -1,6 +1,7 @@
 fn main() {
     cc::Build::new()
         .file("src/cxx.cc")
+        .cpp(true)
         .flag("-std=c++11")
         .compile("cxxbridge03");
     println!("cargo:rerun-if-changed=src/cxx.cc");