dart: ffigen: Add package override

pkgs/development/compilers/dart/package-overrides/default.nix

···
 { callPackage }:
 
 {
+  ffigen = callPackage ./ffigen { };
   flutter_secure_storage_linux = callPackage ./flutter-secure-storage-linux { };
   handy_window = callPackage ./handy-window { };
   matrix = callPackage ./matrix { };

pkgs/development/compilers/dart/package-overrides/ffigen/default.nix

···
+{ lib
+, llvmPackages
+}:
+
+{ ... }:
+
+{ FFIGEN_COMPILER_OPTS ? ""
+, ...
+}:
+
+{
+  FFIGEN_LIBCLANG = lib.getLib llvmPackages.libclang;
+  FFIGEN_COMPILER_OPTS = "-I${FFIGEN_COMPILER_OPTS} ${llvmPackages.clang}/resource-root/include -I${lib.makeSearchPathOutput "dev" "include" [ llvmPackages.clang.libc_dev ]}";
+}