Revert "lib: wrong #define used for IPV6_MINHOPCOUNT"

This reverts commit 54b88cac24f335414caa875b390d2d78ff4bf9e0.

Unfortunately, this breaks the build on systems where linux/in6.h and
netinet/in.h can't both be included, such as Ubuntu 14.04 and Debian
Jessie.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/configure.ac b/configure.ac
index 27014c3..f68d86f 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 linux/in6.h], [], [],
+	netinet6/in6_var.h netinet6/nd6.h], [], [],
 	QUAGGA_INCLUDES)
 
 m4_define([QUAGGA_INCLUDES],dnl
@@ -1092,9 +1092,6 @@
 #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