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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