ncnn: fix build on Darwin

ncnn defaults to static linkage on Darwin against MoltenVK, but static
libraries are not build by default for MoltenVK in nixpkgs, and it’s not
taken as a dependency anyway. Override this behavior by specifying
`Vulkan_LIBRARY` explicitly as `-lvulkan` on Darwin.

pkgs/development/libraries/ncnn/default.nix

@@ -33,7 +33,9 @@ stdenv.mkDerivation rec {
     "-DNCNN_BUILD_TOOLS=0"
     "-DNCNN_SYSTEM_GLSLANG=1"
     "-DNCNN_PYTHON=0" # Should be an attribute
-  ];
+  ]
+  # Requires setting `Vulkan_LIBRARY` on Darwin. Otherwise the build fails due to missing symbols.
+  ++ lib.optionals stdenv.isDarwin [ "-DVulkan_LIBRARY=-lvulkan" ];
 
   nativeBuildInputs = [ cmake ];
   buildInputs = [ vulkan-headers vulkan-loader glslang opencv protobuf ];