texmacs: Use LLVM 17 on Darwin

The current nixos-unstable branches uses Clang 19.1.5 on Darwin per default.
This results in compilation error
"/tmp/nix-build-texmacs-2.1.4.drv-0/TeXmacs/src/Kernel/Containers/hashtree.cpp:97:14:
error: no member named 'contains' in 'hashtree<K, V>'".

pkgs/applications/editors/texmacs/default.nix

@@ -1,6 +1,7 @@
 {
   lib,
   stdenv,
+  llvmPackages_17,
   callPackage,
   fetchurl,
   guile_1_8,
@@ -40,7 +41,11 @@ let
     tex = texliveSmall;
   };
 in
-stdenv.mkDerivation {
+(
+  if stdenv.hostPlatform.isDarwin
+  then llvmPackages_17.stdenv
+  else stdenv
+).mkDerivation {
   inherit pname version;
 
   src = fetchurl {