Add env var to select c++ std lib

Author: jfoote

build.rs

@@ -8,8 +8,19 @@ fn main() {
 
         println!("cargo:rustc-link-search=native={}", custom_lib_dir);
         println!("cargo:rustc-link-lib=static={}", custom_lib_name);
-        /* FIXME: this is assuming a C++ fuzzer, but should be customizable */
-        println!("cargo:rustc-link-lib=stdc++");
+
+        // Allow user to specify use of LLVM or GCC C++ stdlib, or none.
+        // "cargo:rustc-link-lib=stdc++" is used by default for
+        // backward-compatibility.
+        // Note: google/oss-fuzz uses LLVM C++ stdlib as of 15 Jan 2020
+        if let Ok(std_cxx_flag) = ::std::env::var("CUSTOM_LIBFUZZER_STD_CXX") {
+            match std_cxx_flag.as_str() {
+                "c++" => println!("cargo:rustc-link-lib=c++"),
+                "stdc++" => println!("cargo:rustc-link-lib=stdc++"),
+                "none" => (),
+                _ => println!("cargo:rustc-link-lib=stdc++"),
+            }
+        }
     } else {
         let mut build = cc::Build::new();
         let sources = ::std::fs::read_dir("libfuzzer")