Add difftime(3).

libc/Makefile

@@ -357,6 +357,7 @@ time/clock_nanosleep.o \
 time/clock.o \
 time/clock_settime.o \
 time/clock_settimeres.o \
+time/difftime.o \
 time/nanosleep.o \
 time/timens.o \
 time/time.o \

libc/time/difftime.cpp

@@ -0,0 +1,30 @@
+/*******************************************************************************
+
+    Copyright(C) Jonas 'Sortie' Termansen 2013
+
+    This file is part of the Sortix C Library.
+
+    The Sortix C 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 3 of the License, or (at your
+    option) any later version.
+
+    The Sortix C 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 the Sortix C Library. If not, see <http://www.gnu.org/licenses/>.
+
+    time/difftime.cpp
+    Calculate time difference.
+
+*******************************************************************************/
+
+#include <time.h>
+
+extern "C" double difftime(time_t a, time_t b)
+{
+	return (double) (a - b);
+}