Octave: disable optimization of tests for faster compiles/less memory usage

- Filter out all but -g... and -W... flags from OCTAVE_CXXFLAGS
- Use AX_CHECK_COMPILE_FLAG() to check if -O0 is supported, if so
  add to end of OCTAVE_CXXFLAGS to ensure optimization is disabled
- Also run mkoctfile under "env -" to protect it from environment
- Also use more standard-compliant sed expressions
.travis.yml: reduce Octave make jobs to 3

- Allow for more memory per job to prevent gcc failures
- Disabling optimization should give enough speedup to compensate
Revert "Suppress Octave failing the build"

- This reverts commit fea2fc137e2a1db8601e6af922838a71cc47596e.
- Failures were probably due to gcc running out of memory
- Disabling optimization of Octave tests should both increase
  compile times and reduce memory usage, and reducing number
  of make jobs to 3 should also allow for more memory per job