Add nix-flake-c, nix_flake_init_global, nix_flake_settings_new

src/external-api-docs/package.nix

@@ -30,6 +30,7 @@ mkMesonDerivation (finalAttrs: {
       # Source is not compiled, but still must be available for Doxygen
       # to gather comments.
       (cpp ../libexpr-c)
+      (cpp ../libflake-c)
       (cpp ../libstore-c)
       (cpp ../libutil-c)
     ];