dbus_cplusplus: support cross compilation

building examples requires executing a `dbusxx-xml2cpp` helper which is
built earlier in the build process (for the host architecture), with no
easy way to patch it for the correct architecture, so disable examples.

pkgs/development/libraries/dbus-cplusplus/default.nix

···
   nativeBuildInputs = [ pkg-config ];
   buildInputs = [ dbus glib expat ];
 
-  configureFlags = [ "--disable-ecore" "--disable-tests" ];
+  configureFlags = [
+    "--disable-ecore"
+    "--disable-tests"
+  ] ++ lib.optionals (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) [
+    "--disable-examples"
+  ];
 
   meta = with lib; {
     homepage = "https://dbus-cplusplus.sourceforge.net";