deprecate or just delete unused variables?

[Mike Miller]

I ran across a few build-system variables that I think are completely
unused in the current code base. They may have been used at one point
but I don't see any references to them now, other than defining them and
substituting them into various shell scripts, mkoctfile, etc.

  SHLEXT, SHLEXT_VER: These are defined in configure.ac for various
  system types, substituted into build_env and into the octave-config
  helper program, but never actually used for anything practical.

  LIBEXT: This is defined in configure.ac, substituted into the
  octave_config_info function, and used in the values of LIBOCTAVE and
  LIBOCTINTERP. I think this variable itself can be safely made internal
  to configure.ac only.

  LIBPRE, SHLBIN, SHLLIB (and _VER and PRE variants), and SONAME_FLAGS:
  Defined in configure.ac, never appear anywhere else, can definitely be
  deleted.

Any objections to just deleting all of these variables, and making
LIBEXT essentially a private shell variable in configure?

I've taken the liberty of searching for these among all of the Forge
packages. Only SHLEXT appears to be “used”, but that use appears to be
simple boilerplate using octave_config_info to get the value and never
actually use it, easily fixed.

Thanks to bug #48215 for getting me to chase these variables.

-- 
mike