Some Octave patches to reapply

These are some patches to the Octave module I would like to reapply. None of them involve whitespace changes or style cleanups. They make the following changes:
- Fix the failures in the Octave Travis builds by disabling optimisation
- Drop support for Octave versions older than 3.2.0
- Test Octave against version 3.8 (the latest stable series) in the Travis builds
env - should be portable; I've used it in another project which is built on a wide variety of platforms (RedHat/Debian-derived Linux, MacOSX) without problems. It's needed because on some platforms mkoctfile allows its internal variables to be overridden by the environment, e.g. if the user exports CPPFLAGS, then calling mkoctfile -p CPPFLAGS will return the user's CPPFLAGS, rather than the CPPFLAGS needed to build an Octave module.