By default systemd will execute service with root directory
(or home directory for user instance) which might result in
attempts to create files in unexpected place. Let's set it
to 'osmocom' subdir of state directory
(/var/lib for system instance) instead.
I don't really understand why our other spec files only contain
a dependency to autoconf and get away with that (while still using
autoconf-archive from configure.ac), but let's try this.