contributing: add SPDX-License-Identifier tags in lib

[nilason]

Adds SPDX-License-Identifier to all *.c, *.cpp, *.h, *.py, *.sh, Makefile and CMakeLists.txt files in lib directory (where this tag is missing). The lib/external directory is excluded from this.

In some cases, where explicitly stated, SPDX-FileCopyrightText is also added. But that tag will need a follow up PR for the remaining files.

Note, for the files:

• lib/gis/strlcat.c
• lib/gis/strlcpy.c
• lib/vector/dglib/avl.c
• lib/vector/dglib/tavl.c

there is a double license, which I believe (but not positively) should be expressed with AND (e.g. BSD-2-Clause AND GPL-2.0-or-later and LGPL-3.0-or-later AND GPL-2.0-or-later). These files contain mainly external code, with minor additions/modifications by GRASS devs.

Other than this latter issue, this should be pretty straightforward.

I deliberately didn't remove any existing license or copyright text (see https://github.com/OSGeo/grass/blob/main/doc/development/rfc/legal_aspects_of_code_contributions.md).

[nilason]

there is a double license, which I believe (but not positively) should be expressed with AND (e.g. BSD-2-Clause AND GPL-2.0-or-later and LGPL-3.0-or-later AND GPL-2.0-or-later). These files contain mainly external code, with minor additions/modifications by GRASS devs.

Or should these be simply BSD-2-Clause and LGPL-3.0-or-later respectively?

[nilason]

Or should these be simply BSD-2-Clause and LGPL-3.0-or-later respectively?

I updated doing just that.