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