Include m4 dir so autogen.sh won't fail

Running `autogen.sh` without an `m4` directory results in an error.

Git doesn't like empty directories, so I've used the usual hack of dropping an empty `.gitignore` file to make it not empty.

I've also added an exception to the top-level `.gitignore` to allow `m4/.gitignore` but otherwise ignore the contents of the `m4` directory.