Export `CXX` explicitly

This fixes ./configure on old versions of GHC on macOS:

checking whether /nix/store/22a6p8311lg3z4ksrjbj018i7z3vgkva-clang-wrapper-16.0.6/bin/cc supports --target... yes
checking C++ standard library flavour... ./configure: line 12244: /usr/bin/g++ -std=gnu++11: No such file or directory
configure: error: Failed to compile test program

ghc.nix

@@ -262,6 +262,9 @@ hspkgs.shellFor rec {
   shellHook = ''
     # somehow, CC gets overridden so we set it again here.
     export CC=${stdenv.cc}/bin/cc
+    # This prevents `./configure` from detecting the system `g++` on macOS,
+    # fixing builds on some older GHC versions (like `ghc-9.7-start`):
+    export CXX=${stdenv.cc}/bin/c++
     export GHC=$NIX_GHC
     export GHCPKG=$NIX_GHCPKG
     export HAPPY=${happy}/bin/happy