xml-security-c: fix build with gcc 16

nixpkgs now defaults to gcc16, which defaults to gnu++20 standard.

Upstream needs to be patched, but in the meantime we can just use an older
standard.

pkgs/by-name/xm/xml-security-c/package.nix

@@ -27,6 +27,8 @@ stdenv.mkDerivation (finalAttrs: {
     "--with-xalan"
   ];
 
+  env.CXXFLAGS = "-std=gnu++17";
+
   nativeBuildInputs = [
     autoreconfHook
     pkg-config