More generally add the check for possible generator "activation" at
runtime depending on a generator specific check.
The dynamic behavior is currently implemented only for MacOS
and should be fully backward compatible for other system.
Inspired-By Tom Hughes <tomtheengineer@gmail.com>
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.