fix gdb/13620 -- control-c to interrupt gdb command only works once.

if HAVE_SIGSETJMP is not defined, nothing is changed; if it is defined
(as it now is for sysv4 based systems), then the fix is enabled.

gdb/config/xm-sysv4.h

@@ -31,6 +31,9 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */
 
 #define NEED_POSIX_SETPGID
 
+/* SVR4 has sigsetjmp and siglongjmp */
+#define HAVE_SIGSETJMP
+
 /* We have to include these files now, so that GDB will not make
    competing definitions in defs.h.  */
 #include <limits.h>