vimPlugins.codediff-nvim: fix library build on Darwin

Upstream falls back to `brew --prefix libomp` on macOS when
`find_package(OpenMP)` fails, which does not work in the Nix sandbox.

Patch the CMake fallback to use `llvmPackages.openmp` instead.

No extra `libomp.dylib` symlink is needed on Darwin, since the built
library already references `libomp.dylib` from the Nix store.

pkgs/applications/editors/vim/plugins/non-generated/codediff-nvim/default.nix

@@ -7,6 +7,7 @@
   vimPlugins,
   autoPatchelfHook,
   stdenv,
+  llvmPackages,
 }:
 vimUtils.buildVimPlugin rec {
   pname = "codediff.nvim";
@@ -22,8 +23,16 @@ vimUtils.buildVimPlugin rec {
   dependencies = [ vimPlugins.nui-nvim ];
 
   nativeBuildInputs = [ cmake ] ++ lib.optionals stdenv.hostPlatform.isLinux [ autoPatchelfHook ];
-  buildInputs = lib.optionals stdenv.hostPlatform.isLinux [ stdenv.cc.cc.lib ];
+  buildInputs =
+    lib.optionals stdenv.hostPlatform.isLinux [ stdenv.cc.cc.lib ]
+    ++ lib.optionals stdenv.hostPlatform.isDarwin [ llvmPackages.openmp ];
   dontUseCmakeConfigure = true;
+
+  postPatch = lib.optionalString stdenv.hostPlatform.isDarwin ''
+    substituteInPlace libvscode-diff/CMakeLists.txt \
+      --replace-fail 'COMMAND brew --prefix libomp' 'COMMAND echo ${llvmPackages.openmp}'
+  '';
+
   buildPhase = ''
     runHook preBuild
     make