Thanks to "McBen <viertelvor12@gmx.net>" for the patch.
(Did not preserve original commit author information because
we have a push check for first and last name, and do not
accept authors with only an alias...)
This converts the CMake license to a pure 3-clause OSI-approved BSD
License. We drop the previous license clause requiring modified
versions to be plainly marked. We also update the CMake copyright to
cover the full development time range.