New runtime function __enable_vm_eval__() to control use of VM evaluator. * libinterp/parse-tree/pt-vm-eval.cc (F__enable_vm_eval__): New file. New DEFUN for __enable_vm_eval__ protected by #ifdef on configuration variable OCTAVE_ENABLE_VM_EVALUATOR. * libinterp/parse-tree/module.mk: Add pt-vm-eval.cc to build system.

%!function out = bug54490 ()
%!  global k;
%!  k = 1;
%!  out = 3;
%!endfunction

%!test <*54490>
%! global k;
%! k = 2;
%! a = [5, 6];
%! a(k) = bug54490 ();
%! assert (a, [5, 3]);
%! k = 2;
%! a = [5, 6];
%! [a(k)] = bug54490 ();
%! assert (a, [5, 3]);
%! clear -global k;  # cleanup after test