autogen: Create m4/ directory if it doesn't exist

Some versions of aclocal apparently require this:

    autoreconf: running: aclocal --force -I m4
    aclocal: couldn't open directory `m4': No such file or directory
    autoreconf: aclocal failed with exit status: 1

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>