lib: wrong #define used for IPV6_MINHOPCOUNT

The #define IPV6_MINHOPCNT define is never defined on any unix platform.
>From what I can tell the original implementation on the linux platform
was IPV6_MINHOPCNT, when it got accepted into the mainstream kernel
it was transformed into IPV6_MINHOPCOUNT.  Since we test for the
define before attempting to use the code it was silently doing nothing
for a long time.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/configure.ac b/configure.ac
index f68d86f..27014c3 100755
--- a/configure.ac
+++ b/configure.ac
@@ -1072,7 +1072,7 @@
 dnl IPv6 header checks
 dnl ------------------
 AC_CHECK_HEADERS([netinet6/in6.h netinet/in6_var.h netinet/icmp6.h \
-	netinet6/in6_var.h netinet6/nd6.h], [], [],
+	netinet6/in6_var.h netinet6/nd6.h linux/in6.h], [], [],
 	QUAGGA_INCLUDES)
 
 m4_define([QUAGGA_INCLUDES],dnl
@@ -1092,6 +1092,9 @@
 #if HAVE_NETINET6_ND6_H
 # include <netinet6/nd6.h>
 #endif
+#if HAVE_LINUX_IN6_H
+# include <linux/in6.h>
+#endif
 ])dnl
 
 dnl disable doc check