[configure] Refine CLOCK_MONOTONIC test

* configure.ac: Check for the actual CLOCK_MONOTONIC symbol, as clock_gettime
  need not imply that clock type is supported
* lib/zebra.h: use the method given in autoconf docs for sys/time.h
  inclusion
diff --git a/lib/zebra.h b/lib/zebra.h
index 2716460..2dc8451 100644
--- a/lib/zebra.h
+++ b/lib/zebra.h
@@ -55,7 +55,6 @@
 #include <sys/select.h>
 #endif /* HAVE_SYS_SELECT_H */
 #include <sys/stat.h>
-#include <sys/time.h>
 #include <sys/types.h>
 #include <sys/param.h>
 #ifdef HAVE_SYS_SYSCTL_H
@@ -72,7 +71,16 @@
 #include <sys/ksym.h>
 #endif /* HAVE_SYS_KSYM_H */
 #include <syslog.h>
-#include <time.h>
+#ifdef TIME_WITH_SYS_TIME
+# include <sys/time.h>
+# include <time.h>
+#else
+# ifdef HAVE_SYS_TIME_H
+#  include <sys/time.h>
+# else
+#  include <time.h>
+# endif
+#endif /* TIME_WITH_SYS_TIME */
 #include <sys/uio.h>
 #include <sys/utsname.h>
 #ifdef HAVE_RUSAGE