Add timestamps to libdivecomputer.log

Since I learned while trying to implement this that getting sub-second
resolution time in portable C99 is hard (especially for someone who is
used to the comfort of std::chrono and Howard Hinnants date library) the
timer-implemetation from libdivecomputer is now copied to the subsurface
source.

Signed-off-by: Christof Arnosti <charno@charno.ch>

core/timer.h

@@ -0,0 +1,51 @@
+/*
+ * libdivecomputer
+ *
+ * Copyright (C) 2018 Jef Driesen
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+ * MA 02110-1301 USA
+ */
+
+#ifndef DC_TIMER_H
+#define DC_TIMER_H
+
+#include <libdivecomputer/common.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif /* __cplusplus */
+
+#if defined (_WIN32) && !defined (__GNUC__)
+typedef unsigned __int64 dc_usecs_t;
+#else
+typedef unsigned long long dc_usecs_t;
+#endif
+
+typedef struct dc_timer_t dc_timer_t;
+
+dc_status_t
+dc_timer_new (dc_timer_t **timer);
+
+dc_status_t
+dc_timer_now (dc_timer_t *timer, dc_usecs_t *usecs);
+
+dc_status_t
+dc_timer_free (dc_timer_t *timer);
+
+#ifdef __cplusplus
+}
+#endif /* __cplusplus */
+#endif /* DC_TIMER_H */