Fix policy warning with CMake (regarding "no rules to generate")
Fix policy warning with CMake (regarding "no rules to generate")

Fixed by using add_custom_command instead of execute_process,
with an in-place sed command included to strip timestamp just like
previous code.

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master