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 could do that to remove the -Ox flags from OCTAVE_CXX, but the Octave tests are compiled using other flags variables common to all languages, e.g. CXXFLAGS, which might also have -Ox flags. Adding -O0 to OCTAVE_CXX ensures that no optimisation is used. And I've written my own autoconf test for -O0 support which shouldn't require changing the autoconf version.