Add libexif to deps/third_party.

libexif/sources/libexif/exif-system.h

+/*! \file exif-system.h
+ * \brief System specific definitions, not for installation!
+ */
+/*
+ * Copyright (c) 2007 Hans Ulrich Niedermann <gp@n-dimensional.de>
+ *
+ * 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 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 EXIF_SYSTEM_H
+#define EXIF_SYSTEM_H
+
+#if defined(__GNUC__) && (__GNUC__ >= 2)
+# define UNUSED(param) UNUSED_PARAM_##param __attribute__((unused))
+#else
+# define UNUSED(param) param
+#endif
+
+#endif /* !defined(EXIF_SYSTEM_H) */