c-stack: stop using SIGSTKSZ

It’s been proposed to stop making SIGSTKSZ an integer constant:
https://sourceware.org/pipermail/libc-alpha/2020-September/118028.html
Also, using SIGSTKSZ in #if did not conform to current POSIX.
Also, avoiding SIGSTKSZ makes the code simpler and easier to grok.
* lib/c-stack.c (SIGSTKSZ): Remove.
(alternate_signal_stack): Now a 64 KiB array, for simplicity.
All uses changed.

[Retrieved (and backported) from:
https://git.savannah.gnu.org/cgit/gnulib.git/patch/?id=f9e2b20a12a230efa30f1d479563ae07d276a94b]
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

diff -Nura m4-1.4.18.orig/lib/c-stack.c m4-1.4.18/lib/c-stack.c
--- m4-1.4.18.orig/lib/c-stack.c	2021-04-11 19:12:14.086494029 +0200
+++ m4-1.4.18/lib/c-stack.c	2021-04-11 19:48:46.316862760 +0200
@@ -50,15 +50,16 @@
 #if ! HAVE_STACK_T && ! defined stack_t
 typedef struct sigaltstack stack_t;
 #endif
-#ifndef SIGSTKSZ
-# define SIGSTKSZ 16384
-#elif HAVE_LIBSIGSEGV && SIGSTKSZ < 16384
-/* libsigsegv 2.6 through 2.8 have a bug where some architectures use
-   more than the Linux default of an 8k alternate stack when deciding
-   if a fault was caused by stack overflow.  */
-# undef SIGSTKSZ
-# define SIGSTKSZ 16384
-#endif
+
+/* Storage for the alternate signal stack.
+   64 KiB is not too large for Gnulib-using apps, and is large enough
+   for all known platforms.  Smaller sizes may run into trouble.
+   For example, libsigsegv 2.6 through 2.8 have a bug where some
+   architectures use more than the Linux default of an 8 KiB alternate
+   stack when deciding if a fault was caused by stack overflow.  */
+static max_align_t alternate_signal_stack[(64 * 1024
+                                           + sizeof (max_align_t) - 1)
+                                          / sizeof (max_align_t)];
 
 #include <stdlib.h>
 #include <string.h>
@@ -128,19 +129,6 @@
 #if (HAVE_SIGALTSTACK && HAVE_DECL_SIGALTSTACK \
      && HAVE_STACK_OVERFLOW_HANDLING) || HAVE_LIBSIGSEGV
 
-/* Storage for the alternate signal stack.  */
-static union
-{
-  char buffer[SIGSTKSZ];
-
-  /* These other members are for proper alignment.  There's no
-     standard way to guarantee stack alignment, but this seems enough
-     in practice.  */
-  long double ld;
-  long l;
-  void *p;
-} alternate_signal_stack;
-
 static void
 null_action (int signo __attribute__ ((unused)))
 {
@@ -205,8 +193,8 @@
 
   /* Always install the overflow handler.  */
   if (stackoverflow_install_handler (overflow_handler,
-                                     alternate_signal_stack.buffer,
-                                     sizeof alternate_signal_stack.buffer))
+                                     alternate_signal_stack,
+                                     sizeof alternate_signal_stack))
     {
       errno = ENOTSUP;
       return -1;
@@ -279,14 +267,14 @@
   stack_t st;
   struct sigaction act;
   st.ss_flags = 0;
+  st.ss_sp = alternate_signal_stack;
+  st.ss_size = sizeof alternate_signal_stack;
 # if SIGALTSTACK_SS_REVERSED
   /* Irix mistakenly treats ss_sp as the upper bound, rather than
      lower bound, of the alternate stack.  */
-  st.ss_sp = alternate_signal_stack.buffer + SIGSTKSZ - sizeof (void *);
-  st.ss_size = sizeof alternate_signal_stack.buffer - sizeof (void *);
-# else
-  st.ss_sp = alternate_signal_stack.buffer;
-  st.ss_size = sizeof alternate_signal_stack.buffer;
+  st.ss_size -= sizeof (void *);
+  char *ss_sp = st.ss_sp;
+  st.ss_sp = ss_sp + st.ss_size;
 # endif
   r = sigaltstack (&st, NULL);
   if (r != 0)
diff -Nura m4-1.4.18.orig/lib/c-stack.h m4-1.4.18/lib/c-stack.h
--- m4-1.4.18.orig/lib/c-stack.h	2021-04-11 19:12:14.098494042 +0200
+++ m4-1.4.18/lib/c-stack.h	2021-04-11 19:17:42.138848378 +0200
@@ -34,7 +34,7 @@
    A null ACTION acts like an action that does nothing.
 
    ACTION must be async-signal-safe.  ACTION together with its callees
-   must not require more than SIGSTKSZ bytes of stack space.  Also,
+   must not require more than 64 KiB bytes of stack space.  Also,
    ACTION should not call longjmp, because this implementation does
    not guarantee that it is safe to return to the original stack.