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.
This adds the CACHE option to set_property and get_property commands.
This allows full control over cache entry information, so advanced users
can tweak their project cache as desired. The set_property command
allows only pre-defined CACHE properties to be set since others would
not persist anyway.
