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

app/getopt.cpp

@@ -1,22 +1,4 @@
-// ***************************************************************** -*- 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
 
 #if _MSC_VER >= 1400
 # pragma warning(disable : 4996)