Handle C stack overflow caused by too nested Lisp evaluation.
* configure.ac: Check for sigaltstack and related sigaction support. Unconditionally check for sigsetjmp and siglongjmp. (HAVE_STACK_OVERFLOW_HANDLING): Define if we can support it. * src/lisp.h (toplevel) [HAVE_STACK_OVERFLOW_HANDLING]: Declare siglongjmp point to transfer control from SIGSEGV handler. * src/keyboard.c (return_to_command_loop, recover_top_level_message) [HAVE_STACK_OVERFLOW_HANDLING]: New variables. (regular_top_level_message): New variable. (command_loop) [HAVE_STACK_OVERFLOW_HANDLING]: Handle non-local exit from SIGSEGV handler and adjust message displayed by Vtop_level if appropriate. (syms_of_keyboard): DEFVAR Vtop_level_message and initialize new variables described above. * src/sysdep.c [HAVE_SYS_RESOURCE_H]: Include sys/resource.h as such. (stack_grows_down, sigsegv_stack, handle_sigsegv) [HAVE_STACK_OVERFLOW_HANDLING]: New variables and function. (init_sigsegv): New function. (init_signals): Use it. * lisp/startup.el (normal-top-level): Use top-level-message.
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