Use system-cxx-std-lib instead of extra-libraries when available

Just as it says on the tin.

stub/ghc-debug-stub.cabal

@@ -36,7 +36,10 @@ library
   default-language:    Haskell2010
   cxx-sources:         cbits/stub.cpp, cbits/socket.cpp, cbits/trace.cpp
   cxx-options:         -std=gnu++11 -pthread -O3 -g3 -DTHREADED_RTS
-  extra-libraries:     stdc++
+  if impl(ghc > 9.4)
+    build-builds:      system-cxx-std-lib
+  else
+    extra-libraries:   stdc++
   cpp-options: -DTHREADED_RTS
   if flag(trace)
     cxx-options: -DTRACE