cln: fix darwin aarch build

Use gcc environment explicitly. The library manual does not mention
darwin explicitly and recommends only g++ as the compiler. Instead of
trying to patch the source to work with clang, just use g++ as expected.

pkgs/development/libraries/cln/default.nix

···
-{ lib, stdenv, fetchurl, gmp }:
+{ lib, gccStdenv, fetchurl, gmp }:
 
-stdenv.mkDerivation rec {
+gccStdenv.mkDerivation rec {
   pname = "cln";
   version = "1.3.6";