configure: Avoid an implicit int in the IPv6 test

Otherwise, the test fails unconditionally with compilers that
do not support implicit ints (a language feature that was removed with
the C99 language revision).

config/amanda/ipv6.m4

@@ -85,7 +85,7 @@ AC_DEFUN([AMANDA_CHECK_IPV6],
 #include <sys/socket.h>
 #include <errno.h>
 
-main()
+int main(void)
 {
  int aa;
  aa = socket(AF_INET6, SOCK_STREAM, 0);