fix compatibility issue with GNU Hurd

ChangeLog

@@ -18,8 +18,9 @@ Unreleased
   * (feature) --enable-ipv6 is now default
   * (feature) New option '-4' to force IPv4
   * (feature) New option '-6' to force IPv6
-  * A C99 compiler is now required
+  * (bugfix)  Fix compatibility issue with GNU Hurd
-  * Option parsing with optparse (https://github.com/skeeto/optparse)
+  * (other)   A C99 compiler is now required
+  * (other)   Option parsing with optparse (https://github.com/skeeto/optparse)
               Thanks Christopher Wellons!
 
 2017-02-09  David Schweikert  <david@schweikert.ch>

src/fping.c

@@ -597,7 +597,7 @@ int main(int argc, char** argv)
                         perror("setting type of service octet IP_TOS");
                     }
                 }
-#ifdef IPV6
+#if defined(IPV6) && defined(IPV6_TCLASS)
                 if (socket6) {
                     if (setsockopt(socket6, IPPROTO_IPV6, IPV6_TCLASS, &tos, sizeof(tos))) {
                         perror("setting type of service octet IPV6_TCLASS");