In commit 9a5b4eba (All variable accesses should produce watch
callbacks, 2007-05-17) we added a new enumeration value but did not
update the list of strings matching them. Add the missing entry.
Without this, variable_watch prints REMOVED_ACCESS instead of
MODIFIED_ACCESS when set the value of the variable.
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.