[rust] Tell Cargo to link cpp runtime library

https://boringssl-review.googlesource.com/c/boringssl/+/66288
allowed C++ runtime in libssl. The build script of bssl-sys crate
should indicate that the crate requires a C++ runtime. Use
libc++ on MacOS and libstdc++ on other unix like systems by
default. Introduce a new environment variable to configure C++
runtime to use.

Change-Id: Ib445955012126080dd03ad7b650287ea9dde10b0
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/67147
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

rust/bssl-sys/build.rs

diff --git a/rust/bssl-sys/build.rs b/rust/bssl-sys/build.rs
index 91a9f8a..ede6411 100644
--- a/rust/bssl-sys/build.rs
+++ b/rust/bssl-sys/build.rs
@@ -78,6 +78,23 @@
     return Path::new(&crate_dir).join("../../build");
 }
 
+fn get_cpp_runtime_lib() -> Option<String> {
+    println!("cargo:rerun-if-env-changed=BORINGSSL_RUST_CPPLIB");
+
+    if let Ok(cpp_lib) = env::var("BORINGSSL_RUST_CPPLIB") {
+        return Some(cpp_lib);
+    }
+
+    if env::var_os("CARGO_CFG_UNIX").is_some() {
+        match env::var("CARGO_CFG_TARGET_OS").unwrap().as_ref() {
+            "macos" => Some("c++".into()),
+            _ => Some("stdc++".into()),
+        }
+    } else {
+        None
+    }
+}
+
 fn main() {
     let bssl_build_dir = get_bssl_build_dir();
     let bssl_sys_build_dir = bssl_build_dir.join("rust/bssl-sys");
@@ -106,5 +123,9 @@
     );
     println!("cargo:rustc-link-lib=static=rust_wrapper");
 
+    if let Some(cpp_lib) = get_cpp_runtime_lib() {
+        println!("cargo:rustc-link-lib={}", cpp_lib);
+    }
+
     println!("cargo:conf={}", OSSL_CONF_DEFINES.join(","));
 }