zsh: fix sigsuspend probe under autoconf 2.73 / C23

autoconf 2.73 makes AC_PROG_CC pick -std=gnu23, which rejects the K&R
handler in the zsh_cv_sys_sigsuspend probe. The compile failure flips
the probe to "no", defines BROKEN_POSIX_SIGSUSPEND, and the resulting
racy pause() fallback hangs $(...) on a lost SIGCHLD.

Upstream dropped the probe in 8dd271fdec52; that does not apply on 5.9
with the PCRE backports here, so just prototype the handler.

Fixes #513543
Fixes #513019

pkgs/by-name/zs/zsh/fix-sigsuspend-probe-c23.patch

@@ -0,0 +1,17 @@
+Prototype the K&R handler so the probe still compiles under -std=gnu23
+(selected by autoconf 2.73). Upstream removed the probe in 8dd271fdec52,
+which does not apply against 5.9 with the PCRE backports.
+
+https://github.com/NixOS/nixpkgs/issues/513543
+--- a/configure.ac
++++ b/configure.ac
+@@ -2334,8 +2334,7 @@ if test x$signals_style = xPOSIX_SIGNALS; then
+ #include <signal.h>
+ #include <unistd.h>
+ int child=0;
+-void handler(sig)
+-    int sig;
++void handler(int sig)
+ {if(sig==SIGCHLD) child=1;}
+ int main() {
+     struct sigaction act;

pkgs/by-name/zs/zsh/package.nix

@@ -63,6 +63,9 @@ stdenv.mkDerivation (finalAttrs: {
       hash = "sha256-bl1PG9Zk1wK+2mfbCBhD3OEpP8HQboqEO8sLFqX8DmA=";
       excludes = [ "ChangeLog" ];
     })
+    # autoconf 2.73 picks -std=gnu23, breaking the K&R sigsuspend probe and
+    # causing $(...) hangs. Drop with the next zsh release.
+    ./fix-sigsuspend-probe-c23.patch
   ]
   ++ lib.optionals stdenv.cc.isGNU [
     # Fixes compilation with gcc >= 14.