redoconf: clock_gettime() detection needs to depend on -lrt.

More generally, default.func.rc.od wasn't super useful because you
couldn't specify either a header file or library dependencies.  Drop it
and make an explicit clock_gettime.rc.od.  As a bonus, this also checks
that CLOCK_MONOTONIC exists as expected.

Reported-by: Wayne Scott <wsc9tt@gmail.com>

docs/cookbook/c/rc/clock_gettime.rc.od

@@ -0,0 +1,17 @@
+. ./redoconf.rc
+rc_include rc/CC.rc rc/rt.autolib.rc rc/posix.rc
+
+prog="
+#include <time.h>
+
+void test() { clock_gettime(CLOCK_MONOTONIC, NULL); }
+"
+
+rc_appendln LIBS "$LIBRT"
+if rc_compile cc link "$prog"; then
+	rc_replaceln "HAVE_CLOCK_GETTIME" 1
+else
+	rc_undo
+	rc_replaceln "HAVE_CLOCK_GETTIME" ""
+fi
+rc_save