This file exists such that the m4/ directory will be created when cloning the
git repository.

The m4/ directory is not directly used by this project, but libtoolize
populates this directory with files, recommending that the directory be
included in the macro search path for aclocal.

Because autoreconf runs aclocal before libtoolize, this directory will not
exist when autoreconf is run, triggering an error from aclocal.

Creating this directory (and keeping this file in it as a placeholder)
prevents this error.
