Make c-stack use libsigsegv, when available.

Make c-stack use libsigsegv, when available.

* modules/c-stack (Depends-on): Add libsigsegv.
* modules/c-stack-tests (Makefile.am): Link with libsigsegv, if
needed.
* lib/c-stack.c (SIGSTKSZ): Define fallback.
(segv_handler, overflow_handler, c_stack_action)
[HAVE_LIBSIGSEGV && !HAVE_XSI_STACK_OVERFLOW_HEURISTIC]: Add new
implementation when libsigsegv is available, but only when using
the library is necessary.
* m4/c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC): Add
comment, explaining why XSI check fails on Linux.
(gl_PREREQ_C_STACK): Supply LIBCSTACK, LTLIBCSTACK.
* tests/test-c-stack2.sh: Tweak skip message.
* NEWS: Document new link-time requirements.

Signed-off-by: Eric Blake <ebb9@byu.net>