Compatibility fixes for Octave 3.8.

- no longer use OCTAVE_QUIT in MEX files; this seems now only possible in
  oct-files. (Ref #304)
- do not build linsolve.oct on Octave >= 3.8, it is available natively.
- do not add strjoin.m to the patch on Octave >= 3.8, it is available natively.
- default_save_options has been renamed save_default_options.