Import the gethrxtime module from gnulib.

gnulib 9e117ae955a5c6a0406140e62b76c3ef50e3bc2b.

* lib/gethrxtime.c, lib/gethrxtime.h, lib/gettime.c, lib/timespec.c,
lib/timespec.h, lib/xtime.h, m4/clock_time.m4, m4/gethrxtime.m4,
m4/gettime.m4, m4/timespec.m4: New files.
* m4/gnulib-cache.m4, m4/gnulib-comp.m4, lib/Makefile.am: Adjust.

lib/gethrxtime.h

@@ -0,0 +1,37 @@
+/* gethrxtime -- get high resolution real time
+
+   Copyright (C) 2005, 2009-2012 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Paul Eggert.  */
+
+#ifndef GETHRXTIME_H_
+# define GETHRXTIME_H_ 1
+
+# include "xtime.h"
+
+/* Get the current time, as a count of the number of nanoseconds since
+   an arbitrary epoch (e.g., the system boot time).  Prefer a
+   high-resolution clock that is not subject to resetting or
+   drifting.  */
+
+# if HAVE_ARITHMETIC_HRTIME_T && HAVE_DECL_GETHRTIME
+#  include <time.h>
+static inline xtime_t gethrxtime (void) { return gethrtime (); }
+# else
+xtime_t gethrxtime (void);
+# endif
+
+#endif