[configure] Detect support for monotonic clock

Quagga has code to support monotonic clock to avoid issues where
time of day changes. The support was incomplete since it was not being
detected by autoconf.
diff --git a/configure.ac b/configure.ac
index 7cf30a8..c1f6182 100755
--- a/configure.ac
+++ b/configure.ac
@@ -1321,6 +1321,16 @@
  AC_DEFINE(HAVE_RUSAGE,,rusage)],
  AC_MSG_RESULT(no))
 
+dnl --------------------------------------
+dnl checking for clock_time monotonic struct and call
+dnl --------------------------------------
+AC_CHECK_LIB(rt, clock_gettime,
+[AC_DEFINE(HAVE_MONOTONIC_CLOCK,, Have monotonic clock)
+ LIBS="$LIBS -lrt"]
+)
+
+
+
 dnl -------------------
 dnl capabilities checks
 dnl -------------------