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.

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