Use SPDX for licenses (#2122)

* Use SPDX identifier in header files

* Use SPDX identifier in rest of source files

* Fix usage of SPDX for files with 2 licenses

* Add global license file

* Fix compilation

src/futils.cpp

@@ -1,23 +1,5 @@
-// ***************************************************************** -*- C++ -*-
-/*
- * Copyright (C) 2004-2021 Exiv2 authors
- * This program is part of the Exiv2 distribution.
- *
- * 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 2
- * 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, write to the Free Software
- * Foundation, Inc., 51 Franklin Street, 5th Floor, Boston, MA 02110-1301 USA.
- */
-// *****************************************************************************
+// SPDX-License-Identifier: GPL-2.0-or-later
+
 // included header files
 #include "config.h"
 
@@ -27,17 +9,12 @@
 #include "image_int.hpp"
 
 // + standard includes
-#include <sys/stat.h>
-#include <sys/types.h>
-
 #include <algorithm>
 #include <array>
-#include <cerrno>
-#include <cstdio>
-#include <cstring>
 #include <filesystem>
 #include <sstream>
 #include <stdexcept>
+
 #ifdef EXV_HAVE_UNISTD_H
 #include <unistd.h>  // for stat()
 #endif