Preprocessor update

— Fix the interpretation of @#else clause within an @#ifndef
  Closes: #1747
— Fix the interpretation of @#elseif clauses within an @#ifdef or @#ifndef
— PAC model: fixes to M_.pac.MODEL.ar (information about autoregressive part)
  Ref. preprocessor#52
— PAC model: make detection of non-optimizing part more robust
  Ref. preprocessor#50
— Fixes to Julia output