syscall: clock_systimer() is no longer a system call. It has been replaced with the equivalent, standard interface clock() as the system call.

sched/clock:  Move the implementation of clock() from libs/libc/time to sched/clock.  This is necessary because it calls the (now) internal OS function clock_systimer.  clock() is now accessed only via a system call in certain configuratins.
libs/libc/wqueue:  Replace calls to clock_systimer() with calls to the equivalent clock().

sched/clock/Make.defs

@@ -36,7 +36,7 @@
 CSRCS += clock_initialize.c clock_settime.c clock_gettime.c clock_getres.c
 CSRCS += clock_time2ticks.c clock_abstime2ticks.c clock_ticks2time.c
 CSRCS += clock_systimer.c clock_systimespec.c clock_timespec_add.c
-CSRCS += clock_timespec_subtract.c
+CSRCS += clock_timespec_subtract.c clock.c
 
 ifeq ($(CONFIG_CLOCK_TIMEKEEPING),y)
 CSRCS += clock_timekeeping.c

sched/clock/clock.c

@@ -0,0 +1,74 @@
+/****************************************************************************
+ * sched/clock/clock.c
+ *
+ *   Copyright (C) 2015, 2018 Gregory Nutt. All rights reserved.
+ *   Author: Gregory Nutt <gnutt@nuttx.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name NuttX nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+
+#include <time.h>
+
+#include <nuttx/clock.h>
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name:  clock
+ *
+ * Description:
+ *   The clock() function returns the implementation's best approximation to
+ *   the processor time used by the process since the beginning of a
+ *   implementation-defined era related only to the process invocation.
+ *
+ *   To determine the time in seconds, the value returned by clock() should
+ *   be divided by the value of the macro CLOCKS_PER_SEC as defined in <time.h>.
+ *
+ * Input Parameters:
+ *   None
+ *
+ * Returned Value:
+ *   The system time in units of clock ticks is returend.  If the processor
+ *   time used is not available or its value cannot be represented, the
+ *   function will return the value (clock_t)-1.
+ *
+ ****************************************************************************/
+
+clock_t clock(void)
+{
+  return (clock_t)clock_systimer();
+}