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

Submitted upstream: <https://github.com/zmanda/amanda/pull/220>

diff --git a/config/amanda/ipv6.m4 b/config/amanda/ipv6.m4
index 9d76979339fee75a..d91ab95ab97e4ec1 100644
--- a/config/amanda/ipv6.m4
+++ b/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);