Fix Travis autoreconf invocation

Because of: a) a missing --install on the autoreconf call, b) the
non-existence of the m4 directory and c) Debian bug #565663, autoreconf
fails. Commit an empty m4/ dir to handle older versions of autreconf, as
well as an updated .travis.yml, that should fix building under Travis.
Fix compilation for Bison 2.x as well

Commit dba1df9 fixed compilation with newers version of Bison (3.x) but
broken building with 2.x. Fix this in a backwards-compatible way and
while at it fix a couple of Bison 2.x warnings as well.