[cmake] create separate target for bindings

dune/xt/grid/CMakeLists.txt

@@ -15,4 +15,9 @@ add_subdirectory(test EXCLUDE_FROM_ALL )
 if(dune-pybindxi_FOUND)
   dune_pybindxi_add_module(_grid bindings.cc)
   target_link_dune_default_libraries(_grid)
+  if(DUNE_XT_WITH_PYTHON_BINDINGS)
+    add_custom_target(bindings ALL DEPENDS _grid)
+  else()
+    add_custom_target(bindings DEPENDS _grid)
+  endif()
 endif()