build: fix build if siginfo_t lacks si_timerid/si_overrun

* configure.ac (AC_CHECK_MEMBERS): Check for siginfo_t.si_timerid and
siginfo_t.si_overrun.
* signal.c (printsiginfo) [SI_TIMER]: Check HAVE_SIGINFO_T_SI_TIMERID
and HAVE_SIGINFO_T_SI_OVERRUN.
diff --git a/signal.c b/signal.c
index 6f00fc0..a019905 100644
--- a/signal.c
+++ b/signal.c
@@ -453,7 +453,8 @@
 				printsigsource(sip);
 				break;
 #endif
-#ifdef SI_TIMER
+#if defined SI_TIMER \
+ && defined HAVE_SIGINFO_T_SI_TIMERID && defined HAVE_SIGINFO_T_SI_OVERRUN
 			case SI_TIMER:
 				tprintf(", si_timerid=%#x, si_overrun=%d",
 					sip->si_timerid, sip->si_overrun);