Problem: can't test if IPv6 is available in tests

Solution: add helper function is_ipv6_available to testutil.hpp to
test if IPv6 is available on the building platform.
This function will try to open and bind a socket to ::1:*, as it's
the ultimate way of knowing if, at least on the loopback, IPv6 is
enabled.

tests/testutil.hpp

@@ -51,6 +51,7 @@
 #include <string.h>
 
 #if defined _WIN32
+#   include "windows.hpp"
 #   if defined _MSC_VER
 #       include <crtdbg.h>
 #       pragma warning(disable:4996)
@@ -61,6 +62,9 @@
 #   include <signal.h>
 #   include <stdlib.h>
 #   include <sys/wait.h>
+#   include <sys/socket.h>
+#   include <netinet/in.h>
+#   include <arpa/inet.h>
 #endif
 
 //  Bounce a message from client to server and back
@@ -318,5 +322,53 @@ msleep (int milliseconds)
 #endif
 }
 
+// check if IPv6 is available (0/false if not, 1/true if it is)
+// only way to reliably check is to actually open a socket and try to bind it
+int
+is_ipv6_available(void)
+{
+    int rc, ipv6 = 1;
+    struct sockaddr_in6 test_addr;
+
+    memset (&test_addr, 0, sizeof (test_addr));
+    test_addr.sin6_family = AF_INET6;
+    inet_pton (AF_INET6, "::1", &(test_addr.sin6_addr));
+
+#ifdef ZMQ_HAVE_WINDOWS
+    SOCKET fd = socket (AF_INET6, SOCK_STREAM, IPPROTO_IP);
+    if (fd == INVALID_SOCKET)
+        ipv6 = 0;
+    else {
+        setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, (const char *)&ipv6, sizeof(int));
+        rc = setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, (const char *)&ipv6, sizeof(int));
+        if (rc == SOCKET_ERROR)
+            ipv6 = 0;
+        else {
+            rc = bind (fd, (struct sockaddr *)&test_addr, sizeof (test_addr));
+            if (rc == SOCKET_ERROR)
+                ipv6 = 0;
+        }
+        closesocket (fd);
+    }
+#else
+    int fd = socket (AF_INET6, SOCK_STREAM, IPPROTO_IP);
+    if (fd == -1)
+        ipv6 = 0;
+    else {
+        setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &ipv6, sizeof(int));
+        rc = setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, &ipv6, sizeof(int));
+        if (rc != 0)
+            ipv6 = 0;
+        else {
+            rc = bind (fd, (struct sockaddr *)&test_addr, sizeof (test_addr));
+            if (rc != 0)
+                ipv6 = 0;
+        }
+        close (fd);
+    }
+#endif
+
+    return ipv6;
+}
 
 #endif