check SIGINT/SIGTERM/SIGHUP defined before using them

since not all platform define these signal number

Part of #935.

Change-Id: I98f14590ad45d1fe7e61076cce5a76b7874772ea
Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
(cherry picked from commit d4267b66b5550c0e4cfec60439eab1d57a4a0c7a)
Signed-off-by: Bruce A. Mah <bmah@es.net>

src/iperf_api.c

@@ -3862,9 +3862,15 @@ diskfile_recv(struct iperf_stream *sp)
 void
 iperf_catch_sigend(void (*handler)(int))
 {
+#ifdef SIGINT
     signal(SIGINT, handler);
+#endif
+#ifdef SIGTERM
     signal(SIGTERM, handler);
+#endif
+#ifdef SIGHUP
     signal(SIGHUP, handler);
+#endif
 }
 
 /**