Fix MinGW builds

Those need to link against libstdc++.

Author: nox

boring-sys/build/main.rs

@@ -553,7 +553,7 @@ fn get_cpp_runtime_lib(config: &Config) -> Option<String> {
 
     match &*config.target_os {
         "macos" | "ios" | "freebsd" | "android" => Some("c++".into()),
-        _ if config.unix => Some("stdc++".into()),
+        _ if config.unix || config.target_env == "gnu" => Some("stdc++".into()),
         // TODO(rmehra): figure out how to do this for windows
         _ => None,
     }