fix from upstream

commit 4be5608573b2be2e31f3d2a01446e22e1c9d7839
Author: Chet Ramey <chet.ramey@case.edu>
Date:   Wed Jan 27 11:49:50 2021 -0500

    commit bash-20210122 snapshot

diff --git a/lib/readline/signals.c b/lib/readline/signals.c
index 0943c643b27d..ecaf699e9261 100644
--- a/signals.c
+++ b/signals.c
@@ -266,8 +266,10 @@ _rl_handle_signal (int sig)
     case SIGQUIT:
 #endif
 
+#if defined (HAVE_POSIX_SIGNALS)
       if (block_sig)
 	sigprocmask (SIG_BLOCK, &set, &oset);
+#endif
 
       rl_echo_signal_char (sig);
       rl_cleanup_after_signal ();