Try to get IPv6 support, unless --disable-ipv6 is explicitly specified.

configure.in

@@ -92,44 +92,17 @@ AC_DEFINE(ss_family, __ss_family, [KAME hack])
 AC_DEFINE(ss_len, __ss_len, [KAME hack])
 
 CFLAGS="$CFLAGS"
-AC_MSG_CHECKING([whether IPv6 is explicitly enabled])
 AC_ARG_ENABLE(ipv6,
-[  --enable-ipv6		Enable ipv6 (with ipv4) support
-  --disable-ipv6		Disable ipv6 support],
-[ case "$enableval" in
-  no)
-       AC_MSG_RESULT(no)
-       ipv6=no
-       ;;
-  *)   
-       AC_MSG_RESULT(yes)
-       AC_DEFINE(ENABLE_IPV6)
-       ipv6=yes
-       ;;
-  esac ],
-  [
-  AC_MSG_RESULT(no)
-  dnl not specified; let's try
-  AC_MSG_CHECKING([whether IPv6 works anyhow])
-  AC_TRY_COMPILE([ 
-#include <sys/types.h>
-#include <sys/socket.h>
-], 
-[
-main()
-{
- if (socket(AF_INET6, SOCK_STREAM, 0) < 0)
-   exit(1);
- else
-   exit(0);
-}
-],
-  [ 
-  AC_MSG_RESULT(yes)
-  AC_DEFINE(ENABLE_IPV6)
-  ipv6=yes],
-  [AC_MSG_RESULT(no)
-  ipv6=no])])
+	AC_HELP_STRING([--disable-ipv6], [do not try to support IPv6]))
+AC_MSG_CHECKING([whether IPv6 is explicitly disabled])
+if test "$xenable_ipv6" = xyes
+then
+	AC_MSG_RESULT(yes)
+	ipv6=no
+else
+	AC_MSG_RESULT(no)
+	ipv6=yes
+fi
 
 ipv6type=unknown
 ipv6lib=none