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
I think you intended AX_COMPARE_VERSION to be the 3rd argument to AS_IF ? Oherwise AC_MSG_RESULT is called twice and the output looks a bit messy.