> > Fatal error: can't create obj/backend.o: No such file or directory
> To mention this again: The way to tell git (or any other SCM I've
> encountered so far) that an empty directory exists for a reason is to
> create a dummy file in this directory and check that into the SCM. I
> usually use a file named .keep because it then doesn't show up in
> directory listings.
Why should the repository have to remember a build system detail?

The correct way is to have the build system mkdir(2) that directory
if it doesn't exist yet. That also plays nice with attempts to build
in an objdir outside the source dir (perhaps because the source tree
is mounted read-only, as some people prefer).