allow jit compiler to be enabled at run-time * pt-jit.h, pt-jit.cc (Venable_jit_debug, Venable_jit_compiler): New variables. (Fenable_jit_debug, Fenable_jit_compiler): New functions. (jit_info::compile): Make JIT debugging info conditional on Venable_jit_debug. * pt-eval.cc (tree_evaluator::visit_simple_for_command, tree_evaluator::visit_while_command): Make JIT compiling conditional on Venable_jit_compiler. * octave.cc (no_jit_compiler_option, jit_debug_option): New file-scope variables. (JIT_DEBUG_OPTION, NO_JIT_COMPILER_OPTION): New option macros. (log_opts): Add --jit_debug and --no-jit-compiler options. (octave_process_command_line): Handle JIT_DEBUG_OPTION and NO_JIT_COMPILER_OPTION. (octave_initialize_interpreter): Handle jit_debug_option and no_jit_compiler_option.