build: check actually-used BSD link state fields

ifi_link_state missing in OS X. There could be other *BSD's that haven't
implemented it and possibly affects older implementations.

The existing HAVE_BSD_LINK_DETECT configure.ac check is only confirming
the link state detection using ifmediareq.ifm_status found in
<net/if_media.h>. This is the link state detection used in
zebra/ioctl.c. Later, *BSD redefined struct if_data in <net/if.h> and
included link state detection. This is the method used in
zebra/kernel_socket.c

Additional test defined in config.ac to test for member struct
if_data.ifi_link_state defined in <net/if.h> separate from test for
<net/if_media.h> ifmediareq.ifm_status

Fixed #ifdef's in zebra/kernel_socket.c to use the new #define
No impact on older function calls in zebra/ioctl.c

Tested on 64bit OS X 10.7, FreeBSD 9.0 amd64 & i386 (32bit)
using gcc & clang. Tested on linux 64bit.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/configure.ac b/configure.ac
index dcca968..425fe27 100755
--- a/configure.ac
+++ b/configure.ac
@@ -992,6 +992,16 @@
     QUAGGA_INCLUDES)], 
   [], QUAGGA_INCLUDES )
 
+dnl ---------------------------------------------------------------
+dnl Additional, newer way to check link-state using ifi_link_state.
+dnl Not available in all BSD's when ifmediareq available
+dnl ---------------------------------------------------------------
+AC_CHECK_HEADER([net/if.h],
+    AC_CHECK_MEMBERS([struct if_data.ifi_link_state],
+      AC_DEFINE(HAVE_BSD_IFI_LINK_STATE,,[BSD ifi_link_state available]),
+      [], QUAGGA_INCLUDES),
+    ,)
+
 dnl ------------------------
 dnl TCP_MD5SIG socket option
 dnl ------------------------