* lib/c-stack.c: Omit mention of HAVE_SIGALTSTACK, since
the code is used only if a test for sigaltstack worked
in some other way.
* m4/c-stack.m4 (gl_PREREQ_C_STACK): Do not require gl_LIBSIGSEGV;
instead, execute gl_LIBSIGSEGV only if needed (because the XSI
heuristic does not work).
* modules/c-stack (Files): Add m4/libsigsegv.m4, since
we no longer require the libsigsegv module.
(Depends-on): Depend on havelib, not libsigsegv.