python3Packages.pythran: unvendor the xsimd dependency

https://hydra.nixos.org/build/229711355/nixlog/150/tail
https://github.com/NixOS/nixpkgs/pull/246200#issuecomment-1658719714

pythran doesn't seem to support building with external xsimd,
but this source-symlink seems to work fine and it's header-only
C++ library anyway.

pkgs/development/python-modules/pythran/default.nix

···
 , six
 , numpy
 , beniget
+, xsimd
 , isPy3k
 , substituteAll
 }:
···
       gomp = "${if stdenv.cc.isClang then openmp else stdenv.cc.cc.lib}/lib/libgomp${stdenv.hostPlatform.extensions.sharedLibrary}";
     })
   ];
+
+  # xsimd: unvendor this header-only C++ lib
+  postPatch = ''
+    rm -r third_party/xsimd
+    ln -s '${lib.getDev xsimd}'/include/xsimd third_party/
+  '';
 
   propagatedBuildInputs = [
     ply