signal.c: add sigprocmask guard

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58354 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

signal.c

@@ -776,12 +776,16 @@ NORETURN(static void raise_stack_overflow(int sig, rb_thread_t *th));
 static void
 raise_stack_overflow(int sig, rb_thread_t *th)
 {
+#ifdef HAVE_SIGPROCMASK
     sigset_t mask;
+#endif
     clear_received_signal();
+#ifdef HAVE_SIGPROCMASK
     sigemptyset(&mask);
     sigaddset(&mask, sig);
     if (sigprocmask(SIG_UNBLOCK, &mask, NULL))
 	rb_bug_errno("sigprocmask:set", errno);
+#endif
     ruby_thread_stack_overflow(th);
 }