On systems that define it (Sierra and later), tell configure where to
find clockid_t.

https://trac.macports.org/ticket/52546

Upstream-Status: Pending
Signed-off-by: Lawrence Velázquez <larryv@macports.org>

Index: configure
===================================================================
--- configure.orig
+++ configure
@@ -6465,6 +6465,7 @@ cat confdefs.h >>conftest.$ac_ext
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
 $ac_includes_default
+#include <time.h>
 typedef clockid_t ac__type_new_;
 int
 main ()