* progmodes/octave.el (inferior-octave-startup): Store the value

* progmodes/octave.el (inferior-octave-startup): Store the value
of __octave_srcdir__ for octave-source-directories.
(inferior-octave-check-process): New function refactored out of
inferior-octave-send-list-and-digest.
(octave-source-directories)
(octave-find-definition-filename-function): New variables.
(octave-source-directories)
(octave-find-definition-default-filename): New functions.
(octave-find-definition): Improve to find functions implemented in C++.