| Add empty recipes for .cc files generated by protoc. | |
| https://www.gnu.org/software/make/manual/html_node/Empty-Recipes.html | |
| Empty recipes can also be used to avoid errors for targets that will be created as a side-effect of another recipe: if the target does not exist the empty recipe ensures that make won’t complain that it doesn’t know how to build the target, and make will assume the target is out of date. |